SkQP: relax test radial_gradient_precision

motivation: this is a bad test in that low-precision drivers will give
completely different results.

The jitter test might have caught this, but  I ran the jitter test with
the 8888 backend, which has high precision.

Bug: skia:8857 https://bugs.skia.org/8857
Bug: b/127371533
Change-Id: I7111ca34df7e510a1dbb591b1161b32912c82fe0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/200926
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Cherry-Pick: 72614c620d7ae548e7d73a1c7687f2521d04708e
1 file changed