cmd/gomobile: don't dump certificate to test log

The output takes up many lines and obscures other test output from
go test -v.

Change-Id: I0896717db3ce287f9e4b4263451475c19758b0e3
Reviewed-on: https://go-review.googlesource.com/99778
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/cmd/gomobile/cert_test.go b/cmd/gomobile/cert_test.go
index 359d7ad..a4ab860 100644
--- a/cmd/gomobile/cert_test.go
+++ b/cmd/gomobile/cert_test.go
@@ -52,9 +52,7 @@
 			"-i",
 			"-in", sigPath,
 		)
-		out, err := cmd.CombinedOutput()
-		t.Logf("%v:\n%s", cmd.Args, out)
-		if err != nil {
+		if err := cmd.Run(); err != nil {
 			t.Errorf("bad asn.1: %v", err)
 		}
 	}