tests: Move SkUTF tests to their own file.

Change-Id: Ib4816cae8b4f2d4f11bb03ed043c677a17d89ead
Reviewed-on: https://skia-review.googlesource.com/145147
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed