Feature/sections sapna unittests

Fixed unit tests for create_section_form, get_enrollments_without_test_student , remove_section and created separate test files for these. Also added new test casesin these methods for the new utilities 'check_resource_link_id' and 'validate_course_id'
I don't think this test and the one before are both nessesary. I would change one to use the template render with status = 500 and delete the other.