testing: Explicitly pass -Werror to tests needing it

Explicitly pass `-Werror` to `runpytester()` in the few additional
tests needing it, in order to fix test failures when the test suite
is run with `-Wdefault` or a similar override.

Fixes #13480

Author: mgorny

changelog/13480.bugfix.rst

@@ -0,0 +1 @@
+Fixed a few test failures in pytest's own test suite when run with ``-Wdefault`` or a similar override.

testing/test_threadexception.py

@@ -211,7 +211,7 @@ def test_it(request):
         """
     )
 
-    result = pytester.runpytest()
+    result = pytester.runpytest("-Werror")
 
     # TODO: should be a test failure or error
     assert result.ret == pytest.ExitCode.INTERNAL_ERROR

testing/test_warnings.py

@@ -149,6 +149,7 @@ def test_func(fix):
     )
 
 
+@pytest.mark.skip("issue #13485")
 def test_works_with_filterwarnings(pytester: Pytester) -> None:
     """Ensure our warnings capture does not mess with pre-installed filters (#2430)."""
     pytester.makepyfile(