Previously we were still calling GoogleTest directly. This change
switches the test main funtions to call the standalone harness. Only
affects standalone and shouldn't affect the tests in Chromium or on
the current test setup.

Bug: angleproject:3162
Change-Id: Ia5a5a73d47c0b47b4df8f54cd6df71da2d878847
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1998661
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>