Add pytest-randomly to randomize test execution

[matrss]

Right now we run tests in normal order and in reverse order in CI. This finds some issues, but not all of them. Ideally we would run tests in a random order to avoid any implicit dependencies between tests.

Right now the test suite is not in a state in which this would be possible, AFAICT.

[matrss]

I think the main issues with getting the test suite to pass in random order are the same that made problems on macOS in #2249:

• Test_WMS.test_files_changed
• Test_WMS.test_gallery
• The entirety of test_sideview.py, test_topview.py and test_wms_control.py

This means that we should be able to use a random test order in the workflow that is introduced in that PR, without much additional effort.