This source set target lets other targets import the test utils without
needing to export them. They get built into angle_util. They also get
compiled into the various tests and samples. The change also fixes
export issues.

Moves some of the GN logic into the util/ subfolder.

Bug: angleproject:3162
Bug: chromium:1030192
Change-Id: If99d201092ad8541c0de60b3bd893ac9b5875270
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1968259Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>