This test is to ensure the upcoming Vulkan implementation can coalesce
all the calls into a single render pass clear.

Bug: angleproject:2361
Change-Id: I816907b3512715c1d3217689413b6c8e9842a37a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1544973
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>