Cube maps go through a separate path on D3D so we need to test them.

BUG=angleproject:1815

Change-Id: Ifb7a85d7e2750f25bce382fdd7a00062d23b3573
Reviewed-on: https://chromium-review.googlesource.com/597213
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>