Fix secure shell test.
Change the string we check for on exit to only check that we exited with a 0 exit code. This prevents flakes in this test with minor changes to the exit string.

TBR=nirnimesh@chromium.org
BUG=chromium-os:34796

Review URL: https://codereview.chromium.org/10993040

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@158888 0039d316-1c4b-4281-b951-d872f2087c98