Also introduce a test_utils directory that contains helpers used for all
types of tests.

BUG=angleproject:892

Change-Id: I9e1bff895020ffd3a109162283971a290a1098bd
Reviewed-on: https://chromium-review.googlesource.com/270198Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>