testing: data race between parallel panicking and normal subtest [1.14 backport]

[gopherbot]

@ianlancetaylor requested issue #37551 to be considered for backport to the next 1.14 minor release.

@gopherbot Please backport to 1.14. This is a regression that was newly introduced in 1.14.

[gopherbot]

Change https://golang.org/cl/224257 mentions this issue: [release-branch.go1.14] testing: fix data race between parallel subtests

[cagedmantis]

Approved. It's a regression in 1.14 without a known workaround.

[gopherbot]

Closed by merging ab9d037 to release-branch.go1.14.