fix: pass pytest common CourseEnrollmentTests

[Carlos-Muniz]

As part of #36, several tests have been discovered that are not passing in tutor, but passing in devstack. The following tests may not be run by tutor, or may be failing in tutor.

common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::

• test_upgrade_deadline_for_non_upgradeable_enrollment_1_paid_bootcamp
• test_upgrade_deadline_for_non_upgradeable_enrollment_2_masters
• test_upgrade_deadline_for_non_upgradeable_enrollment_3_paid_executive_education
• test_upgrade_deadline_for_non_upgradeable_enrollment_4_professional
• test_upgrade_deadline_for_non_upgradeable_enrollment_5_verified
• test_upgrade_deadline_for_non_upgradeable_enrollment_7_no_id_professional
• test_upgrade_deadline_for_non_upgradeable_enrollment_8_credit

For more information about the tests that passed, failed, skipped, or never ran, see the spreadsheet with all the tests.

[Carlos-Muniz]

Closing because all tests passed when retested.

common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_save_deletes_cached_enrollment_status_hash PASSED [  5%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_6_professional PASSED [ 11%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_users_enrolled_in_all PASSED [ 16%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_3_masters PASSED [ 22%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_1_paid_bootcamp PASSED [ 27%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_with_schedule_and_professional_mode PASSED [ 33%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_instructor_paced PASSED [ 38%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline PASSED [ 44%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_5_no_id_professional PASSED [ 50%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_8_verified PASSED [ 55%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_enrollment_status_hash_cache_key PASSED [ 61%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_2_executive_education PASSED [ 66%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_with_schedule PASSED [ 72%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_users_enrolled_in_active_only PASSED [ 77%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_generate_enrollment_status_hash PASSED [ 83%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_4_paid_executive_education PASSED [ 88%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_upgrade_deadline_for_non_upgradeable_enrollment_7_credit PASSED [ 94%]
common/djangoapps/student/tests/test_models.py::CourseEnrollmentTests::test_enrollments_not_deleted PASSED [100%]

[Carlos-Muniz]

This test does pass in tutor. See #155 for more details.