Disable more flaky mojo_apptests

Randomly getting "Unrecognized --gtest_list_tests output", which makes no sense to me. Example:

https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/178678

Only seems to affect network_service_apptests.

BUG=569367
TBR=msw@chromium.org
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/1684923002 .

Cr-Commit-Position: refs/heads/master@{#374528}