An upcoming CL (https://skia-review.googlesource.com/c/7995/ (Remove asTextureRef from SkSpecialImage & update effects accordingly)) modifies some untested portions of the SkMagnifierImageFilter.
This adds a test to prevents regressions.
Change-Id: I9fa406f699e39fa393212e7f63a457b015b36edb
Reviewed-on: https://skia-review.googlesource.com/8023
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
