testing: in TestRacyOutput, ensure that the Run calls complete before cleanup

testing: in TestRacyOutput, ensure that the Run calls complete before cleanup

Also tweak the failure message added in CL 352349.

Updates #18741.
Updates #48515.

Change-Id: I46ed84c6f498d7a68414cc3dab3c1cd55da69aa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/451215
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Changkun Ou <mail@changkun.de>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>