Fix for Issue-27: TestResult models failing to save

This is a fix for issue: #27 

Similar to issue #16 , TestResult models were validating the presence of a submission id. However, when tests are run on request by the student, a submission id is not required. Therefore, submission ids on these models should also be optional.

This fix modifies the following:
* Remove submission id validation on the TestResult model
* Remove tests that test if TestResult models are validating the presence of a submission id