feat: Add SayHello.

Add SayHello function for test.

go.mod

@@ -0,0 +1,3 @@
+module hellogomod
+
+go 1.24.4

hello.go

@@ -0,0 +1,5 @@
+package hellogomod
+
+func SayHello(name string) string {
+	return "Hello, " + name + "!"
+}

hello_test.go

@@ -0,0 +1,12 @@
+package hellogomod
+
+import "testing"
+
+func TestSayHello(t *testing.T) {
+	s := SayHello("yuki")
+	want := "Hello, yuki!"
+
+	if s != want {
+		t.Errorf("return %s, want %s", s, want)
+	}
+}