Templates make it easier to override parts of test configs. Instead
of if statements we can use forwarded variables to override specific
settings. This refactor makes it easier to integrate the "KHR" tests.

Bug: angleproject:3353
Change-Id: Ia6e9c6d01864e5253db5d28dc087bf954aee226c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1612297Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>