This is a reland of 69e46942

Original change's description:
> Vulkan: Add semaphores test to VulkanExternalImageTest
> 
> 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>

Bug: angleproject:3289
Change-Id: Idce1e8944b72445c92be9ad08d8c5869ca0b7df2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2197735Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>