Make _warn_external correctly report warnings arising from tests.

[anntzer]

Before the PR:

$ pytest lib/matplotlib/tests/test_cbook.py::test_is_hashable
<elided>
lib/matplotlib/tests/test_cbook.py::test_is_hashable
  .../site-packages/_pytest/python.py:166: MatplotlibDeprecationWarning:
  The is_hashable function was deprecated in Matplotlib 3.1 and will be removed in 3.3. Use isinstance(..., collections.abc.Hashable) instead.
    testfunction(**testargs)

-- Docs: https://docs.pytest.org/en/latest/warnings.html

After

$ pytest lib/matplotlib/tests/test_cbook.py::test_is_hashable
<elided>
lib/matplotlib/tests/test_cbook.py::test_is_hashable
  .../lib/matplotlib/tests/test_cbook.py:22: MatplotlibDeprecationWarning:
  The is_hashable function was deprecated in Matplotlib 3.1 and will be removed in 3.3. Use isinstance(..., collections.abc.Hashable) instead.
    assert cbook.is_hashable(s)

lib/matplotlib/tests/test_cbook.py::test_is_hashable
  .../lib/matplotlib/tests/test_cbook.py:25: MatplotlibDeprecationWarning:
  The is_hashable function was deprecated in Matplotlib 3.1 and will be removed in 3.3. Use isinstance(..., collections.abc.Hashable) instead.
    assert not cbook.is_hashable(lst)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[NelleV]

It would be nice to have unit tests.

[anntzer]

sure, here it is.