improve GOOS=tamago test coverage

Author: abarisani

src/cmd/internal/testdir/testdir_test.go

@@ -116,6 +116,10 @@ func Test(t *testing.T) {
 	goExperiment = env.GOEXPERIMENT
 	goDebug = env.GODEBUG
 
+	if goos == "tamago" {
+		t.Skipf("skipping: unsupported")
+	}
+
 	common := testCommon{
 		gorootTestDir: filepath.Join(testenv.GOROOT(t), "test"),
 		runoutputGate: make(chan bool, *runoutputLimit),