Make sweep gradient zero-x test more forgiving

Change-Id: I9153d93a2aeb314fdf822bed8e726e7ad9b26801
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/739856
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
1 file changed