testing: surprising interaction of subtests with TempDir [1.16 backport]

[gopherbot]

@ianlancetaylor requested issue #46624 to be considered for backport to the next 1.16 minor release.

@gopherbot Please open backport to 1.16.

[gopherbot]

Change https://golang.org/cl/378914 mentions this issue: [release-branch.go1.16] testing: drop unusual characters from TempDir directory name

[dmitshur]

Approving as a fix for testing behavior. Only 1.16 needs the backport, as 1.17 already has this fix.

[gopherbot]

Closed by merging 8144f1d to release-branch.go1.16.