test2json: default to "pass" when the test doesn't report failures

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>

src/cmd/internal/test2json/test2json.go

@@ -284,7 +284,7 @@ func (c *converter) flushReport(depth int) {
 func (c *converter) Close() error {
 	c.input.flush()
 	c.output.flush()
-	e := &event{Action: "fail"}
+	e := &event{Action: "pass"}
 	if c.result != "" {
 		e.Action = c.result
 	}

src/cmd/internal/test2json/testdata/benchshort.json

@@ -4,4 +4,4 @@
 {"Action":"output","Output":"# but to avoid questions of timing, we just use a file with no \\n at all.\n"}
 {"Action":"output","Output":"BenchmarkFoo   \t"}
 {"Action":"output","Output":"10000 early EOF"}
-{"Action":"fail"}
+{"Action":"pass"}

src/cmd/internal/test2json/testdata/empty.json

+{"Action":"pass"}