When a test has a TestMain that doesn't run any tests (doesn't invoke
m.Run), `go test` passes, but `go test -json` reports a "fail" event
though the exit code is still 0.

This CL fixes test2json to behave similarly to `go test` in such cases -
no output from the test is taken as "pass" by default, not as "fail".

Fixes #31969

Change-Id: I1829d40fc30dc2879e73974fac416f6a34212ccd
Reviewed-on: https://go-review.googlesource.com/c/go/+/192104
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>