The previous behavior would cause inproper parsing;
- the bot=bot1 flag would add 'bot1': ['defaulttests'] to the dictionary.
- the testfilter=test1 flag would append 'test1' to the list, causing the value
  to look like 'bot1': ['defaulttests', 'test1'].

I think on the long run the best is to keep --testfilter as a user interface but
not on the try server. This effectively stops sending testfilter=foo to the
server.

Add unit test to verify it's working properly. Refactor a bit to make unit
testing simpler.

R=petermayo@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/9599012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@125405 0039d316-1c4b-4281-b951-d872f2087c98