Make WangsFormula_* tests more tolerant of rsqrt approximation

TBR=michaelludwig@google.com

Change-Id: I01e9bd833b604628487a9f345d6fae8340f2848d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319197
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed