Fixed errors when running tests in random order

[radarhere]

Helps #5526

With pytest-randomly installed, Tests/test_decompression_bomb.py can fail, because we are currently setting Image.MAX_IMAGE_PIXELS in three methods, and not resetting it until the tests in TestDecompressionBomb are done in teardown_class.

This PR instead resets it after each method.

[radarhere]

Running py.test-3.8 Tests/test_imagefile.py Tests/oss-fuzz/test_fuzzers.py, test_truncated_with_errors and test_broken_datastream_with_errors can fail with "DID NOT RAISE <class 'OSError'>".

I've added code to reset enable_decompression_error.

[hugovk]

In normal order, pytest passes.

With pytest-reverse, it crashes at Tests/test_qt_image_qapplication.py:

$ pytest --reverse
======================================================= test session starts ========================================================
platform darwin -- Python 3.9.5, pytest-6.2.4, py-1.9.0, pluggy-0.13.1
rootdir: /Users/hugo/github/Pillow, configfile: setup.cfg, testpaths: Tests
plugins: cov-2.12.1, hypothesis-6.9.2, requests-mock-1.8.0, rerunfailures-9.1.1, flaky-3.7.0, mockito-0.0.4, xdist-2.2.1, reverse-1.2.0, testmon-1.0.3, timeout-1.4.2, forked-1.3.0
collected 2812 items

Tests/oss-fuzz/test_fuzzers.py ............................................................................................. [  3%]
............................................................................................................................ [  7%]
............................................................................................................................ [ 12%]
............................................................................................................................ [ 16%]
............................................................................................................................ [ 20%]
............................................................................................................................ [ 25%]
............................................................................................................................ [ 29%]
....................................................................................................................         [ 33%]
Tests/test_webp_leaks.py .                                                                                                   [ 33%]
Tests/test_util.py ......                                                                                                    [ 34%]
Tests/test_tiff_ifdrational.py ....                                                                                          [ 34%]
Tests/test_tiff_crashes.py .............ss                                                                                   [ 34%]
Tests/test_shell_injection.py ....                                                                                           [ 34%]
Tests/test_sgi_crash.py ...........                                                                                          [ 35%]
Tests/test_qt_image_toqimage.py .                                                                                            [ 35%]
Tests/test_qt_image_qapplication.py %

With pytest-randomly, pytest also crashes at Tests/test_qt_image_qapplication.py

$ pytest
======================================================= test session starts ========================================================
platform darwin -- Python 3.9.5, pytest-6.2.4, py-1.9.0, pluggy-0.13.1
Using --randomly-seed=824403459
rootdir: /Users/hugo/github/Pillow, configfile: setup.cfg, testpaths: Tests
plugins: cov-2.12.1, hypothesis-6.9.2, requests-mock-1.8.0, rerunfailures-9.1.1, flaky-3.7.0, randomly-3.8.0, mockito-0.0.4, xdist-2.2.1, reverse-1.2.0, testmon-1.0.3, timeout-1.4.2, forked-1.3.0
collected 2812 items

Tests/test_file_gimppalette.py ..                                                                                            [  0%]
Tests/test_pickle.py ...............                                                                                         [  0%]
Tests/test_file_jpeg2k.py .....s....................sss....                                                                  [  1%]
Tests/test_box_blur.py ..............                                                                                        [  2%]
Tests/test_image_rotate.py ..............                                                                                    [  2%]
Tests/test_file_apng.py ....................                                                                                 [  3%]
Tests/test_psdraw.py ...                                                                                                     [  3%]
Tests/test_image_point.py ....                                                                                               [  3%]
Tests/test_image_array.py ..                                                                                                 [  3%]
Tests/test_lib_pack.py ....................................                                                                  [  5%]
Tests/test_file_eps.py .................                                                                                     [  5%]
Tests/test_image_getbands.py .                                                                                               [  5%]
Tests/test_image_filter.py .........                                                                                         [  6%]
Tests/test_qt_image_qapplication.py %

As that file isn't touched here, skipping it with --ignore=Tests/test_qt_image_qapplication.py means both reverse and random pass. 👍

Would it be worth adding random or reverse (reverse being more "stable") to one of the CI jobs?

[radarhere]

Ok, I've created PR #5542.