Improve perf_test_runner script
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Olli Etuaho <38ae6c0e2772a20e8e989f80cb8c99d274e44242@nvidia.com>

mlfarrell/angle,MSOpenTech/angle,domokit/waterfall,domokit/waterfall,ecoal95/angle,mikolalysenko/angle,ecoal95/angle,ecoal95/angle,ppy/angle,mlfarrell/angle,ppy/angle,mikolalysenko/angle,mlfarrell/angle,mikolalysenko/angle,ppy/angle,mikolalysenko/angle,MSOpenTech/angle,mlfarrell/angle,ecoal95/angle,ppy/angle,MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle