This removes a GN naming conflict between util/system_utils and
common/system_utils. This conflict was preventing us from adding
unit tests to utils' version of system_utils. Since these functions are
only useful to tests and samples rename them test_utils for simplicity.

Will enable further development of ANGLE's standalone testing harness.

Bug: angleproject:3162
Change-Id: I9e34fb69f96c5de6dc2453fce4148a0f285e15ed
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1825268Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>