Added 2 tests in StateChangeTest.cpp to validate the behavior and
reproduce the issue I saw in dEQP.

Bug: angleproject:2479

Change-Id: I29c3da0474ec2b13a10fc266284cb19a07675da2
Reviewed-on: https://chromium-review.googlesource.com/1025951
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>