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>

go.mod

@@ -1,12 +1,16 @@
 module example
 
-go 1.21.2
+go 1.21
 
-require github.com/gin-gonic/gin v1.9.1
+require (
+	github.com/gin-gonic/gin v1.9.1
+	github.com/stretchr/testify v1.8.3
+)
 
 require (
 	github.com/bytedance/sonic v1.9.1 // indirect
 	github.com/chenzhuoyu/base64x v0.0.0-20221115062448-fe3a3abad311 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.2 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
@@ -20,6 +24,7 @@ require (
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.0.8 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.11 // indirect
 	golang.org/x/arch v0.3.0 // indirect