Vulkan: Allow no validation layers in ExternalImageTest. Similarly to common test init, these specific tests init the Vulkan resources with validation layers when they might not be available. Fix this by checking for the appropriate define. Bug: b/168744561 Change-Id: I93dd3bac839297ced07cb2c4cbbcf64db2e65a1f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419041 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>