Update dEQP build config to use GN templates. 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/+/1612297 Reviewed-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>