[mus] Disable badly flaking apptest
[mus] Disable badly flaking apptest

See the bug for details. The test itself is fine, but problems
with the apptest framework are for whatever reason exaggerated
when running this test.

BUG=592313
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/1770753002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379514}

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