cmd/test2json: fix processing of --- BENCH: output

cmd/test2json: fix processing of --- BENCH: output

If a benchmark calls b.Log without failing (without b.Error/b.Fatal/b.FailNow)
then that turns into output very much like a test passing,
except it says BENCH instead of PASS.
Benchmarks failing say FAIL just like tests failing.

Fixes #23346.

Change-Id: Ib188e695952da78057ab4a13f90d49937aa3c232
Reviewed-on: https://go-review.googlesource.com/86396
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>