[test.py] Make test.py quickly skip tests that it isn't going to run
[test.py] Make test.py quickly skip tests that it isn't going to run

The current behavior is to - for instance - create all multitests in a
suite (e.g. language_2) up front, despite having a test-selector that
restricts it a lot.

This CL skips tests faster (i.e. before creating multitests).

Example:

Now:

$ time tools/test.py -m release -a x64 -c dartk --strong language_2/lalala
Test configuration: dartk_vm_release_x64_strong


real    0m0.598s
user    0m0.760s
sys     0m0.116s

Before:

$ time tools/test.py -m release -a x64 -c dartk --strong language_2/lalala
Test configuration: dartk_vm_release_x64_strong


real    0m1.796s
user    0m2.156s
sys     0m0.344s

Change-Id: Ib3c254d4e18c8ed4e6c66f7e25f384ce6b652f02
Reviewed-on: https://dart-review.googlesource.com/44061
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>

dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk