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 +1,3 @@
+Modified a few self-tests to explicitly pass ``-Werror``
+to ``runpytester()``, in order to fix test failures when the test suite
+is 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_unraisableexception.py

@@ -257,7 +257,7 @@ def test_it():
     )
 
     with _disable_gc():
-        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

@@ -169,7 +169,7 @@ def test_my_warning(self):
                     assert True
     """
     )
-    result = pytester.runpytest()
+    result = pytester.runpytest("-Werror")
     result.stdout.fnmatch_lines(["*== 1 passed in *"])