Correct tests for slug serialization

Change-Id: Ibd1a84e3b8f5f900a49cd8e224c1d6aa81128d8d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1398636
Reviewed-by: Brandon Wilson <bwils@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>