cmd/test2json: handle running command and processing output

[rsc]

The official way to get JSON output from a test binary foo.test is

./foo.test -test.v 2>&1 | go tool test2json

but that's annoying on Unix and harder on Windows. Probably go tool test2json should accept a command to run and process:

go tool test2json ./foo.test -test.v

[zolotov]

go tool test2json ./foo.test -test.v is the solution for cross-platform running.

Still, I would prefer something like ./foo.test -test.json. It's consistent with the rest of the testing arguments and it's definitely debuggable with Delve. I'm not sure whether Delve will work fine with the go tool test2json tool.

[gopherbot]

Change https://golang.org/cl/81555 mentions this issue: cmd/test2json: add ability to run test binary

[zolotov]

For some reason my comment was just ignored. Filed #23033