Some tests require a lot of memory and they fail on buildbots if they run
in parallel with other tests of the same kind.
We try to limit this by disabling multiple vairants of the same tests,
assuming this would stop the failures in most of the cases.

BUG=

Review-Url: https://codereview.chromium.org/2653603002
Cr-Commit-Position: refs/heads/master@{#42741}