This removes default testfilter value ['defaulttests'] from polluting properties
everywhere.

This also implies that default testfilter is no testfilter or empty list, which
should have equivalent meening in try recipes.

R=phajdan.jr@chromium.org,nodir@chromium.org
BUG=487822

Review URL: https://codereview.chromium.org/1423483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297228 0039d316-1c4b-4281-b951-d872f2087c98