Enforces that each builder and builder:test association must be in its own list item in PRESUBMIT.py/GetPreferredTrySlaves().

If a user run git try -t foo, trychange.py will now take the list of all the
slaves, will skip any :compile builder, and apply the test filter to them.

Currently, git try -t foo on a chromium checkout throws an exception because of
PRESUBMIT.py files containing test specification.

R=petermayo@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/9664015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@125975 0039d316-1c4b-4281-b951-d872f2087c98