cmd/go: script test hanging with blocked output pipes

[bcmills]

greplogs -l -e 'goroutine \d+ \[chan receive, \d+ minutes\]:\n(?:.+\n\t.+\n)*cmd/go_test\.waitOrStop' --since=2022-01-01
2022-06-06T15:42:19-47e34ca/windows-amd64-longtest
2022-05-26T22:25:23-55c42c2/windows-amd64-longtest
2022-05-20T23:37:43-0a1a092/linux-amd64-longtest
2022-05-20T23:37:43-0a1a092/windows-amd64-longtest
2022-05-20T23:05:38-c3470ca/linux-amd64-longtest
2022-05-20T23:05:38-c3470ca/windows-amd64-longtest
2022-05-20T22:30:37-2b0e457/linux-amd64-longtest
2022-05-20T22:30:37-2b0e457/windows-amd64-longtest
2022-05-20T21:54:20-b580670/linux-amd64-longtest
2022-05-20T21:54:20-b580670/windows-amd64-longtest
2022-05-20T21:32:57-7ec6ef4/linux-amd64-longtest
2022-05-20T21:32:57-7ec6ef4/windows-amd64-longtest
2022-02-03T22:55:24-248ad85/netbsd-amd64-9_0

[bcmills]

This is some script test hanging while waiting on the subprocess's output pipes to close, with the output pipes stuck open.

This is quite possibly a bug in the go command, but it will be much more straightforward to diagnose once #50436 is implemented. (A workaround in the interim is possible but more complex, because we would either have to run our own pipe-copying goroutines or direct all of the stderr and stdout output pipes for script tests to files.)

I plan to implement #50436 for Go 1.20, so milestoning to Go 1.20.

[bcmills]

Duplicate of #50187.