Add a more substantial test that uses semaphores to
VulkanExternalImageTest.

It's still just a clear in GL, which exposed a bug that a staged clear
wasn't be flushed by a call to glSignalSemaphoreEXT.

Bug: angleproject:3289
Change-Id: Id938eaf2c8c20cb0ae9fb948fbfcf3448980b577
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2195684
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>