Run mojo_apptests on debug Linux as well.

Any test which runs on the main waterfall should run on debug and release. Otherwise a dcheck could be hit which wouldn't show up on the main waterfall, but would fail on the trybots which use release-with-asserts.

BUG=478244

Review URL: https://codereview.chromium.org/1705883002

Cr-Commit-Position: refs/heads/master@{#376028}