Fix for Issue-16: Test results not being displayed in the student interface

This is a fix for issue: https://github.com/JeremyMerkur/Markus/issues/16

In commit https://github.com/JeremyMerkur/Markus/commit/95fd189d7707d44b080edcba409db619d9c9cae8 Daniel made it a requirement for TestScriptResults to contain submission ids.  However, not all tests should contain submission ids. For example, tests run by request of student before submission will not have associated submission ids. 

Because of this new requirement, when trying to save a TestScriptResult object, validation was failing and Rails refused to save the model.

This fix makes submission ids on the TestScriptResult object optional rather then required. It also removes tests that validate the presence of a submission id on a TestScriptResult object.