Moves a lot of the code out of the gni file into tests/BUILD.gn. In
preparation for auto-generating the gni file as a simple array of trace
test names.

Bug: angleproject:4590 
Change-Id: Iaa2b7872dc6e705d59108fa7532594bdc65d365a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2168359
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>