testing: summary and test output interleaved [1.14 backport] #40880
Description
@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.