🎨 Clear cache after test touching GlobalConfiguration

The cache is shared between tests and when using Redis, even between
parallel test processes. Certain global configuration options can
affect other tests.

Backport-of: 2e7a611

src/openforms/registrations/contrib/email/tests/test_backend.py

@@ -104,6 +104,11 @@ def setUpTestData(cls):
         cls.form = form
         cls.fd = fd
 
+    def setUp(self):
+        super().setUp()
+
+        self.addCleanup(GlobalConfiguration.clear_cache)
+
     def test_submission_with_email_backend(self):
         submission = SubmissionFactory.from_components(
             completed=True,