Fix platform unittests.

Follow-up to 87db4ff1f4d76eecb8f19d62f4c04279d56bf229, which added
suppressions to unittests.status and a special case for Android to the
ThreadLocalStorageTest, both of which are unneccessary and should be
handled differently for the GTest based unittests.

BUG=v8:3706
LOG=n
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/757913002

Cr-Commit-Position: refs/heads/master@{#25495}