Vulkan: Fix EGLBlobCacheTest.
Vulkan: Fix EGLBlobCacheTest.

Several runs of the same test in a row would cause a failure if the
cache would get hit. Fix the flakiness by resetting the cache between
runs.

Also cleans up some of the variable naming and adds a stream output
for pretty test errors.

Fixed while working on the standalone test harness.

Bug: angleproject:3162
Change-Id: I64da8a37eedf562860e3e5409cbf6fb08e81dfa4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2047417
Reviewed-by: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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