When creating a test with lldbinline.MakeInlineTest(), the reported inspect.getfile(test.__class__) is lldbtest.pyc, meaning any .categories file will be ineffective for those tests. Check for the test_filename first, which inline tests will set.
Additionally, raise an error with the starting dir if .categories is not found. This makes the problem more obvious when it occurs: when the test is separated from the test framework tree.