Add default selector exclution flag to test.dart

Add default selector exclution flag to test.dart

This can be used like this:
tools/test.py -mrelease --exclude-suite=co19,pkg
to run the default selector tests but not run pkg and co19 tests.

Why am I doing this:
We want to keep broad testing on developer machines, but want to allow testing resources to have a better and more logic distribution on the bots, e.g., not run pkg tests on the vm bots.

R=whesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35424

[Bill Hesse, 2014-04-25 08:48:39]

lgtm

[ricow1, 2014-04-25 14:42:36]

Committed patchset #1 manually as r35424 (presubmit successful).