feat: Add SayHi.

Add a new function(lib upgrade).

hello.go

@@ -3,3 +3,7 @@ package hellogomod
 func SayHello(name string) string {
 	return "Hello, " + name + "!"
 }
+
+func SayHi(name string) string {
+	return "Hi, " + name + "!"
+}

hello_test.go

@@ -2,6 +2,15 @@ package hellogomod
 
 import "testing"
 
+func TestSayHi(t *testing.T) {
+	s := SayHi("yuki")
+	want := "Hi, yuki!"
+
+	if s != want {
+		t.Errorf("return %s, want %s", s, want)
+	}
+}
+
 func TestSayHello(t *testing.T) {
 	s := SayHello("yuki")
 	want := "Hello, yuki!"