Fix cases of variable shadowing in /tests/.

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

Change-Id: Ib8b92275c5da71c4ee48540d434f3afdc45f4067
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438819
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
13 files changed