[release-branch.go1.14] testing: fix data race between parallel subtests

This CL fixes a race condition if there are two subtests, and
one finishing but the other is panicking.

For #37551
Fixes #37959

Change-Id: Ic33963eb338aec228964b95f7c34a0d207b91e00
Reviewed-on: https://go-review.googlesource.com/c/go/+/221322


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 93a9561b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/224257


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Changkun Ou <euryugasaki@gmail.com>