add test (and fix) for glyphsToUnichars

Bug: skia:
Change-Id: I2defcea7e9a37bdd203acc4ca97fc47a7f57cd43
Reviewed-on: https://skia-review.googlesource.com/c/176221
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed