go: add more tests (+return instead of exit)

run.go

@@ -65,7 +65,7 @@ func run() error {
 
 	if *license {
 		fmt.Fprintln(os.Stderr, licenseHeader)
-		os.Exit(0)
+		return nil
 	}
 
 	printHeader()

run_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"flag"
+	"testing"
+)
+
+func TestPrintLicense(t *testing.T) {
+	t.Parallel()
+	t.Log("print license and exit")
+
+	if err := flag.Set("license", "true"); err != nil {
+		t.Fatal("failed to set license flag")
+	}
+
+	err := run()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPrintHeader(t *testing.T) {
+	t.Parallel()
+	printHeader()
+}