We already reject recipes in the RemoteSettingsExperimentLoader if they have reference features not found in the feature manifest. However, there are several tests that reference bogus features which should really be failing. We now refuse to enroll silently in these recipes. Tests will now fail loudly if they use the `NimbusTestUtils.enroll()` or `NimbusTestUtils.enrollWithFeatureConfig()` APIs. Differential Revision: https://phabricator.services.mozilla.com/D248561