Telemetry mse_cases: check correctly for test_done property.

This is related to https://codereview.chromium.org/93783007/

BUG=None
NOTRY=true

Review URL: https://codereview.chromium.org/113323004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240196 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240196