Fix issue where fixtures would lose the decorated functionality

Fix #3774

testing/example_scripts/acceptance/fixture_mock_integration.py

@@ -0,0 +1,17 @@
+"""Reproduces issue #3774"""
+
+import mock
+
+import pytest
+
+config = {"mykey": "ORIGINAL"}
+
+
+@pytest.fixture(scope="function")
+@mock.patch.dict(config, {"mykey": "MOCKED"})
+def my_fixture():
+    return config["mykey"]
+
+
+def test_foobar(my_fixture):
+    assert my_fixture == "MOCKED"