The test utils are enhanced to allow redirecting stderr to stdout.  This
is in preparation for a change that makes the test runner capture stderr
together with stdout.  Currently, on failure logs originating from
UNIMPLEMENTED() and other such macros are not captured.

Bug: angleproject:6077
Change-Id: I7a3c6c4732a59dac3ff0cc20a7835d5ed6f0f22e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2976183Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>