runtime: add more debug info to flaky TestNumGoroutine

This has been flaking on the new OpenBSD 5.8 builders lately:
https://storage.googleapis.com/go-build-log/808270e7/openbsd-amd64-gce58_61ce2663.log
(as one example)

Add more debug info when it fails.

Updates #14107

Change-Id: Ie30bc0c703d2e9ee993d1e232ffc5f2d17e65c97
Reviewed-on: https://go-review.googlesource.com/18938
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/runtime/proc_test.go

@@ -350,7 +350,7 @@ func TestNumGoroutine(t *testing.T) {
 	n := runtime.NumGoroutine()
 
 	if nstk := strings.Count(string(buf), "goroutine "); n != nstk {
-		t.Fatalf("NumGoroutine=%d, but found %d goroutines in stack dump", n, nstk)
+		t.Fatalf("NumGoroutine=%d, but found %d goroutines in stack dump: %s", n, nstk, buf)
 	}
 }