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/+/2047417Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>