[test] Limit "unittests" suite to default variant.

[test] Limit "unittests" suite to default variant.

This changes the unittests suite to be run in only the default variant,
because unit tests are (and should be) written to be independent from
global flags.

R=machenbach@chromium.org

Committed: https://crrev.com/e81ee90018283555f811719f6ae0091f6198c526
Cr-Commit-Position: refs/heads/master@{#28054}

[Michael Starzinger, 2015-04-24 12:01:46]

PTAL. This reduces runtime of the "unittests" suite from ~50sec to ~15sec on my
machine. The following is the situation ...

Before:
[00:51|% 100|+ 9432|-   0]: Done

After:
[00:13|% 100|+ 2358|-   0]: Done

[Michael Achenbach, 2015-04-24 21:42:20]

lgtm. On the bots we pass an explicit --no-variants to the test driver for
executing the unittests already, but this is better.

[Michael Starzinger, 2015-04-27 07:58:16]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-04-27 07:58:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1067863007/1

[commit-bot: I haz the power, 2015-04-27 08:20:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-27 08:20:38]

Patchset 1 (id:??) landed as
https://crrev.com/e81ee90018283555f811719f6ae0091f6198c526
Cr-Commit-Position: refs/heads/master@{#28054}