Add test

go.mod

@@ -0,0 +1,3 @@
+module hello
+
+go 1.17

hello.go

@@ -2,6 +2,10 @@ package main
 
 import "fmt"
 
-func main() {
+func Hello() string {
-	fmt.Println("Hello, world")
+	return "Hello, world"
+}
+
+func main() {
+	fmt.Println(Hello())
 }

hello_test.go

@@ -0,0 +1,12 @@
+package main
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	got := Hello()
+	want := "Hello, world"
+
+	if got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+}