The define that controlled the debug layer preference was mistakenly
checking a Vulkan define. When the Vulkan define was moved into the
Vulkan back-end then the layers became disabled for all back-ends.

Fix this by using a layered check in the Vulkan back-end and restoring
the asserts enabled check to ShouldUseDebugLayers.

Suppresses some tests that regressed the layers after they were broken.

Bug: angleproject:3987
Bug: angleproject:3991
Change-Id: I6aafcff6d9e19866c2324663ef4d4550e97fcfb8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1852706Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>