[libc] [UnitTest] Give UnitTest gtest like colors

Summary:
This is a quality of life change to make it a little nicer to look at, NFC.
This patch makes the RUN and OK lines green and FAILED lines red to match gtest's output.

Reviewers: sivachandra, gchatelet, PaulkaToast

Reviewed By: gchatelet

Subscribers: MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D75103