fix bug in signal marking notpassing whitelisted certs

[melvinsoft]

Fix RED-2361

Problem description: Certificates have a Django signal, when a certificate object is created and has a passing status (downloadable or generating) but the course or the grading change, and it ends up in a non passing grade for the learner, the certificate change automatically to notpassing.
That's fine except when there is an Exception certificate created from the instructor dashboard, then it should never change to notpassing.

[OmarIthawi]

Thanks @melvinsoft. Please fix the flaky upstream test:

_ RecalculateSubsectionGradeTest.test_persistent_grades_enabled_on_course_2___split___3__38_ _

self = <lms.djangoapps.grades.tests.test_tasks.RecalculateSubsectionGradeTest testMethod=test_persistent_grades_enabled_on_course_2___split___3__38_>
default_store = 'split', num_mongo_queries = 3, num_sql_queries = 38

    @ddt.data(
        (ModuleStoreEnum.Type.mongo, 1, 38),
        (ModuleStoreEnum.Type.split, 3, 38),
    )
    @ddt.unpack
    def test_persistent_grades_enabled_on_course(self, default_store, num_mongo_queries, num_sql_queries):
        with self.store.default_store(default_store):
            self.set_up_course(enable_persistent_grades=True)
            with check_mongo_calls(num_mongo_queries):
                with self.assertNumQueries(num_sql_queries):
>                   self._apply_recalculate_subsection_grade()

The code looks great.

[coveralls]

Coverage increased (+0.004%) to 42.042% when pulling 83a99a8 on maxi/fix-RED-2361 into 5dfb8b4 on main.