testing: correct comments on runCleanup

testing: correct comments on runCleanup

The comment on runCleanup states "If catchPanic is true ...", but
there is no catchPanic argument or variable. This was introduced
in CL 214822, which introduced the panicHandling type. The code was
updated during code review, but the comment was missed.

Change-Id: Id14c5397e7a026bfdf98ea10ecb1e4c61ce2f924
Reviewed-on: https://go-review.googlesource.com/c/go/+/538695
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>