improve GOOS=tamago test coverage

Author: abarisani

src/net/dial_test.go

@@ -955,7 +955,7 @@ func TestDialListenerAddr(t *testing.T) {
 	// To address both of those problems, we open a tcp4-only localhost port, but
 	// then dial the address string that the listener would have reported for a
 	// dual-stack port.
-	ln, err := Listen("tcp4", ":0")
+	ln, err := Listen("tcp4", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
 	}

src/net/http/transport_test.go

@@ -3394,6 +3394,9 @@ func testProxyForRequest(t *testing.T, tt proxyFromEnvTest, proxyForRequest func
 		t.Errorf("%v: got error = %q, want %q", tt, g, e)
 		return
 	}
+	if url == nil && tt.want == "<nil>" {
+		return
+	}
 	if got := fmt.Sprintf("%s", url); got != tt.want {
 		t.Errorf("%v: got URL = %q, want %q", tt, url, tt.want)
 	}