Disable mojo apptests since they seem to be hanging often.

Disable mojo apptests since they seem to be hanging often.

Looking at http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel?numbuilds=200, there seems to be a lot of flakiness with the test suites. During the last 200 try runs, the test ran about 75 times, with about 10 of them flaking. When it flakes, the test suite appears to hang for 20 minutes after which it's killed by the trybot (normally it finishes very fast).

Example failures:
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/92073
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/92061
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/92014
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91998
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91994
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91973
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91932
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91906
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/91890

BUG=478244
TBR=msw@chromium.org

Review URL: https://codereview.chromium.org/1157923002

Cr-Commit-Position: refs/heads/master@{#331306}