GTTF: Add a basic new unit test launcher.

It launches batches of unit tests in a subprocess for better stability.

BUG=236893
R=maruel@chromium.org, satorux@chromium.org

Review URL: https://codereview.chromium.org/23132002

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 720cedab7dd4555d1d0eba2982e17cac751fb571