Add more documentation on testing infrastructure. Adds more info on the amount of testing ANGLE runs and our various configs. Adds a few other notes and cross-links. Bug: angleproject:1944 Change-Id: I76e46783bd6a3f84d3e170bd30ea2e6d57911143 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2105715 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>