Fix additional cases of variable shadowing in /tests/.

If we manage to fix all the existing cases of variable shadowing, we
could enable -Wshadow.

Change-Id: Ia92c86a902acc6864f3252431faeb157628b509e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439156
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
9 files changed