[release-branch.go1.15] cmd/test2json: do not emit a final Action if the result is not known

If we are parsing a test output, and the test does not end in the
usual PASS or FAIL line (say, because it panicked), then we need the
exit status of the test binary in order to determine whether the test
passed or failed. If we don't have that status available, we shouldn't
guess arbitrarily — instead, we should omit the final "pass" or "fail"
action entirely.

(In practice, we nearly always DO have the final status, such as when
running 'go test' or 'go tool test2json some.exe'.)

Updates #40132
Fixes #40805

Change-Id: Iae482577361a6033395fe4a05d746b980e18c3de
Reviewed-on: https://go-review.googlesource.com/c/go/+/248624


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 1b86bdbd)
Reviewed-on: https://go-review.googlesource.com/c/go/+/248725