Mojo: Add mojo_shell_tests to the Linux bots.

(It seems to be passing on the, uh, Linux Trusty FYI bots, which should
be close enough.)

R=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=279472

Review URL: https://codereview.chromium.org/331993008

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@279801 4ff67af0-8c30-449e-8e8b-ad334ec8d88c