The GN test() template now generates bin/run_ wrappers for
targets; this CL ensures that they'll properly declare when
the wrappers need to use Xvfb on Linux.

Bug: chromium:816629
Change-Id: I4f227eab3306f737f77bbe3341039d710184db96
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2370337Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>