testing: summary and test output interleaved [1.14 backport]

[gopherbot]

@bcmills requested issue #40771 to be considered for backport to the next 1.14 minor release.

@gopherbot, please backport to Go 1.14 and 1.15. This appears to be a regression in the 1.14.6 point release, and there is no apparent workaround for users who require JSON-formatted test output.

[gopherbot]

Change https://golang.org/cl/252638 mentions this issue: [release-branch.go1.14] testing: flush test summaries to stdout atomically when streaming output

[bcmills]

Discussion on #40771 (comment) confirms that the change in question does seem to fix the reported issue, and we haven't had any reports of it regressing any other behavior on the main branch during the 1.16 cycle. I think we should aim to get this into the next minor release.

[cagedmantis]

This has been approved because it's a serious issue without a workaround.

[gopherbot]

Closed by merging 26c6b51 to release-branch.go1.14.