FileTest::testPatchIndividual() and FileTest::testDeleteIndividual() failing on 8.6

Much like #2966363: CommentTest::testPatchIndividual() failing on 8.6 broke due to access control handler changes in core in 8.6 in #2950125: Add helpful reason for 'update' and 'delete' access not being allowed to CommentAccessControlHandler, the same happened in #2950129: Add helpful reason for 'update' and 'delete' access not being allowed to MediaAccessControlHandler (fixed in #2969493: MediaTest::testPatchIndividual() and MediaTest::testDeleteIndividual() failing on 8.6) and now again in #2950127: Add helpful reason for 'update' and 'delete' access not being allowed to FileAccessControlHandler. Hence this issue.