Remove unnecessary line break

The line breaks were introduced due to the parameter names being too
long, but after some refactoring the line breaks are not needed anymore.
Refactor method signatures for reusability

The current use cases for the count update methods can live with the
current signature, but after assigning graders to criteria, the same
update needs to be performed with a modified signature.
Specify Criterion.update_assigned_groups_counts

And remove the old Test::Unit + Shoulda tests.
Specify Grouping.update_criteria_coverage_counts

The old tests in Test::Unit and Should are migrated to RSpec therefore
can be removed.
Fix mocha stubs for count update methods

The signature of the methods changed, so the stubbing calls need update
as well.
Migrate TA assignment tests to RSpec
Merge pull request #1568 from zhangsu/refactor_assigning_graders

Refactor assigning graders