chore: update Go version and add dependencies

- Update the Go version from `1.21.2` to `1.21`
- Add `github.com/stretchr/testify` as a required dependency
- Add `github.com/davecgh/go-spew` as an indirect dependency
- Add `github.com/pmezard/go-difflib` as an indirect dependency
- Add a new test file `router/router_test.go`
- Implement a test for the `PingRoute` function in `router/router_test.go`

Signed-off-by: appleboy <appleboy.tw@gmail.com>

router/router_test.go

@@ -0,0 +1,20 @@
+package router
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPingRoute(t *testing.T) {
+	router := New()
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("GET", "/ping", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, 200, w.Code)
+	assert.Equal(t, "pong", w.Body.String())
+}