Add test cases which exercise the logic for updating the marks available
and marks obtained.

Update the service method so that they are transactional and reset the
transient 'updated' and 'created' flags after reloading entites to
ensure accurate update/create reporting.

- Add validation to /import payload
- Move import logic to service bean
- Track whether entities have been created or update
- Report number of created and updated entities as return value for the
  import endpoint
- Add some test coverage to exercise the validators

Implement core business logic for working with Student, Test, and TestResult.