[release-branch.go1.15] testing: fix Cleanup race with Logf and Errorf

Updates #40908
Fixes #41034

Change-Id: I25561a3f18e730a50e6fbf85aa7bd85bf1b73b6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/250078


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 00a053bd)
Reviewed-on: https://go-review.googlesource.com/c/go/+/250617


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Michał Łowicki <mlowicki@gmail.com>