tests: Avoid modifying '17 prebuild shared' test dir

[heftig]

Tests can tread on each other's toes when parallelism is high enough.

In this case, test_prebuilt_shared_lib creates an object file in the 17 prebuilt shared test dir.

test_prebuilt_shared_lib_rpath_same_prefix uses shutil.copytree to copy that same test dir to a temporary location.

If the former test cleans up its object file while copytree is running, the copy can fail with a fatal ENOENT shutil.Error.

Use copy_srcdir to prevent this from happening.

[eli-schwartz]

Use copy_srcdir in all tests that modify the testdir (that I spotted) to prevent this from happening.

I think that this should only be an issue for test_prebuilt_shared_lib, not the others. There are other tests as well which modify the source directory, e.g. test_prebuilt_static just above -- but that's fine because it's the only test that uses that directory so it won't stomp on the other test via parallelism.

In general, any test that has self.addCleanup() calls is modifying the testdir in some manner

[heftig]

Reduced to just the one case that triggers the crash.