Simple REST Base Web Server with Unit Tests

Question

I am building out a simple web server in Go. My goal is to expand on this framework in a different application I am building.

Here is my github repo https://github.com/jmacnc/gowebserver

Is what I have here using best practices?

I am specifically looking for feedback on the tests, and how I am connecting to different databases (testing and main application). As seen below...

//Handler for posting a new user
func (d *MongoDB) PostUser(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    //Retrieving request body
    var user User
    _ = json.NewDecoder(r.Body).Decode(&user)
    //Posting Company
    err := d.PostUserToDB(user)
    //Error Handling
    if err != nil {
        fmt.Println(err)
        w.WriteHeader(500)
    }
}

//Populating a new user in the database
func (d *MongoDB) PostUserToDB(user User) error {
    _, err := d.db.Collection("users").InsertOne(context.Background(), user)
    if err != nil {
        return err
    }
    return nil
}

func main() {
    //Connects to the database
    d , _ := ConnectToDB()
    //Creates mux router/multiplexer
    r := mux.NewRouter()
    //Handle Functions
    r.HandleFunc("/user", d.PostUser).Methods("POST")
    //Starts web server
    //Prints a string saying that the app is running...
    fmt.Println("Application Running...")
    log.Fatal(http.ListenAndServe(LOCALHOST, r))
}