Reduces much of the code duplication by adding some new helper methods.
The helpers allow combining test parameters similarly to how the
GoogleTest Combine() and Values() generators work. They are more
general and work by returning collections of test parameters instead
of combining generator functions.

Also updates the GLMark2 benchmark runner to use the new methods.

Bug: angleproject:3630
Change-Id: Ibc10f9afb401e119d67a7119974a1a8d9b5abb60
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057353
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>