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'
The mocks in the method call are out of order also in this case you are calling the mock for get_list_sections canvas_replacement, I think it would be good to be consistent with the other tests and call it get_section_replacement.