Add submission metadata to assertRequiredFilesPresent

[ChrisMayfield]

During the CS 149 planning meeting today, we had the idea to combine these two tests into one. The code in test_submission_count could be added to assertRequiredFilesPresent.

    @weight(0)
    def test_submission_count(self):
        """Submission count"""
        try:
            meta = json.load(open("/autograder/submission_metadata.json"))
        except FileNotFoundError:
            print("WARNING: metadata not found (please notify your instructor)")
            return  # This error should happen only when testing the autograder

        count = 1 + len(meta["previous_submissions"])
        stamp = datetime.fromisoformat(meta["created_at"])
        stamp += timedelta(hours=3)  # Pacific to Eastern
        stamp = stamp.strftime("%b %d at %H:%M:%S")
        print(f"Submission {count} ({stamp})")

    @required()
    @weight(0)
    def test_submitted_files(self):
        """Check submitted files"""
        self.assertRequiredFilesPresent([FILENAME])

In addition, assertRequiredFilesPresent could enforce a submission limit:

    @required()
    @weight(0)
    def test_submission_count(self):
        """Submission count"""
        try:
            meta = json.load(open("/autograder/submission_metadata.json"))
        except FileNotFoundError:
            print("WARNING: metadata not found (please notify your instructor)")
            return  # This error should happen only when testing the autograder

        count = 1 + len(meta["previous_submissions"])
        print(f"Submission {count} of {MAX_SUBMISSIONS}")
        if count > MAX_SUBMISSIONS:
            self.fail("Limit exceeded. Please click the Submission History button "
                      "and activate the submission you would like us to grade.\n")