Improve error message when TestCase functions use a parametrized fixture

Fix #2535

testing/example_scripts/unittest/test_parametrized_fixture_error_message.py

@@ -0,0 +1,13 @@
+import pytest
+import unittest
+
+
+@pytest.fixture(params=[1, 2])
+def two(request):
+    return request.param
+
+
+@pytest.mark.usefixtures("two")
+class TestSomethingElse(unittest.TestCase):
+    def test_two(self):
+        pass