---
$ ./python -Wd -m test -j0 test_warnings
Run tests in parallel using 6 child processes
0:00:01 [1/1] test_warnings
(...)
Warning -- warnings.filters was modified by test_warnings
1 test altered the execution environment:
    test_warnings
Total duration: 0:00:02
---

The problem are these two lines in test_warnings/__init__.py:
---
py_warnings = support.import_fresh_module('warnings', blocked=['_warnings'])
c_warnings = support.import_fresh_module('warnings', fresh=['_warnings'])
---

Each fresh "import warnings" calls _processoptions(sys.warnoptions) which can change warning filters.

Attached patch saves/restores warnings.filter to fix the resource warning from the test suite.

Note: the warning is not emited if tests are run sequentially (without the -jN option).