Examples:

• [Impeller] Speed up vulkan startup time by re-using existing vulkan context. #166784
• [Impeller] prefer 24 bit depth buffer format on vulkan backend. #166854

It seems like these tests might be just slow enough that presubmit machines (is it a different pool than post submit?) can't quite finish them in time.

FYI @bkonyi

I added a revert label to #166698