Implementation with test.
Implementation & tests for CommandLineArguments
Implementation for CommandLineTestRunner

(Remove temporary workarounds from CommandLineArguments).
Partial implementation for TestRegistry

(Remove temporary workarounds from CommandLineTestRunner).
Remove pointless getters from TestRegistry
Add remaining changes

Behavior is as before; e.g. if both a group name and
a test name is given, both must match. This is not what
we need ultimately; group names and test names should be
possible to add freely and all should be run.
Finalize new implementation.