Mark slow tests discovered by manual audit of reported durations.

Slow tests will run first, which will lead to higher test runner throughput.

Also some tests have flaky timeouts occasionally.

BUG=chromium:601468

Change-Id: If9aeb5422e79ff25b867b01c50386a5fdebeb494
Reviewed-on: https://chromium-review.googlesource.com/455736Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43819}