- Moves platform-specific platform tests to test-platform.cc.
- Moves all tests into v8::internal namespace.

Bug: chromium:756050
Change-Id: Ic67d6b84dda07a9d5f6a0cc6489bfae90c52c0f2
Reviewed-on: https://chromium-review.googlesource.com/739983
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49012}