Improve perf_test_runner script 1) Make it possible to run the script from any working directory 2) Abort the run if angle_perftests is executing multiple test cases 3) Print some information on which exe and test is running 4) Print the test runner output in case there was an error BUG=angleproject:596 Change-Id: If30c2455dc39b0a776df03e6c1dda2ced90d73ba Reviewed-on: https://chromium-review.googlesource.com/319090 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>