Add support for testing with Mesa + Zink. To run a trace test with zink, use --use-gl=zink. Bug: angleproject:7689 Change-Id: I70073756f903db1c224fe6175d55be4cad637aca Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3899382 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>