Several VulkanExternalImageTest* tests are timing out locally,
but not consistently.

Bug: angleproject:5721
Change-Id: Iffb6d319bb31206af3287a70259a47857b64f434
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2937736Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>