This test was taking 14-20 seconds in release mode. Likely because of
the way we were issuing a ReadPixels and "std::endl" to a stream for
every pixel of a 256x256 Framebuffer. Instead issue a single ReadPixels
call and no stream flushes. Now runs in less than 500ms.

Helps prevent timeouts when running tests multi-process. Also enables
the test on D3D9.

Bug: angleproject:3162
Change-Id: I27aad773040d6b6668cbde003802345e01044a7d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2047414Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>