cmd/test2json: clarify usage instructions

[cespare]

(Copied from a comment I left on https://golang.org/cl/76872 after it was merged.)

I tried using the new test2json tool and found it initially confusing. Here it says

// Usage:
//
//	go test ... | go tool test2json [-p pkg] [-t]
//	./test.out 2>&1 | go tool test2json [-p pkg] [-t]

Should that recommend go test -v? Without -v, it says the test failed even when it passed:

$ go test strings | go tool test2json
{"Action":"output","Output":"ok  \tstrings\t(cached)\n"}
{"Action":"fail"}

cc @rsc

[gopherbot]

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