Improve test to make sure svn status comes out clean.

TEST=improved unit tests and scm.SVN.Revert() should now do the right thing
BUG=none

Review URL: http://codereview.chromium.org/6650011

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@77391 0039d316-1c4b-4281-b951-d872f2087c98