Use value-parameterized tests, loading a full test list from
case files.

BUG=angleproject:998

Change-Id: Iff65a2d722e9d105e22649b17a18cc74dccea2b1
Reviewed-on: https://chromium-review.googlesource.com/274421Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>