Enhance unittest support

See https://github.com/D-Programming-Language/dmd/pull/3518 for much more details.

### Default druntime tester ###
Only updated to ignore ```@disabled``` tests and use the new format. No user-visible changes.

One useful change would be calling every single test in a ```try``` block, not the complete module. This we testing would continue after a test failed in the module. Thoughts?

### Example test runner ###
This is unfortunately outdated/ won't compile, but it shows the idea:
http://dpaste.dzfl.pl/046ed6fb
http://dpaste.dzfl.pl/2780939b

### See also ###
DMD pull request:
https://github.com/D-Programming-Language/dmd/pull/3518