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 <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>