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'
Added it. I think this was the very first test case that  was added for sections tool. Surprised that ithis slipped  thru the numerous review without being  noticed.