Vulkan: Allow no validation layers in ExternalImageTest.
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle