Make the exception tests more robust

This should ensure that the tests work with Pytest 8 onwards. The
issue appears to be that __subclasses__ "returns a list of all those
references still alive." which could include the RequestRedirect. If
it does include RequestRedirect the tests will fail as it requires an
argument to be constructed. Note this test is not meant for
RequestRedirect.

tests/test_exceptions.py

@@ -7,7 +7,7 @@
 from werkzeug import exceptions
 from werkzeug.datastructures import Headers
 from werkzeug.datastructures import WWWAuthenticate
-from werkzeug.exceptions import HTTPException
+from werkzeug.exceptions import default_exceptions, HTTPException
 from werkzeug.wrappers import Response
 
 
@@ -138,7 +138,7 @@ def test_retry_after_mixin(cls, value, expect):
 @pytest.mark.parametrize(
     "cls",
     sorted(
-        (e for e in HTTPException.__subclasses__() if e.code and e.code >= 400),
+        (e for e in default_exceptions.values() if e.code and e.code >= 400),
         key=lambda e: e.code,  # type: ignore
     ),
 )
@@ -158,7 +158,7 @@ def test_description_none():
 @pytest.mark.parametrize(
     "cls",
     sorted(
-        (e for e in HTTPException.__subclasses__() if e.code),
+        (e for e in default_exceptions.values() if e.code),
         key=lambda e: e.code,  # type: ignore
     ),
 )