Example of testing Go HTTP servers using httptest.Server.

main.go

package main

import (
	"log"
	"myserver"
	"net/http"
)

const addr = "localhost:12345"

func main() {
	mux := http.NewServeMux()
	handler := &myserver.MyHandler{}
	mux.Handle("/favicon.ico", http.NotFoundHandler())
	mux.Handle("/", handler)
	log.Printf("Now listening on %s...\n", addr)
	server := http.Server{Handler: mux, Addr: addr}
	log.Fatal(server.ListenAndServe())
}

myserver.go

package myserver

import (
	"fmt"
	"net/http"
	"sync"
)

type MyHandler struct {
	sync.Mutex
	count int
}

func (h *MyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	var count int
	h.Lock()
	h.count++
	count = h.count
	h.Unlock()

	fmt.Fprintf(w, "Visitor count: %d.", count)
}

myserver_test.go

package myserver

import (
	"fmt"
	"io/ioutil"
	"net/http"
	"net/http/httptest"
	"testing"
)

func TestMyHandler(t *testing.T) {
	handler := &MyHandler{}
	server := httptest.NewServer(handler)
	defer server.Close()

	for _, i := range []int{1, 2} {
		resp, err := http.Get(server.URL)
		if err != nil {
			t.Fatal(err)
		}
		if resp.StatusCode != 200 {
			t.Fatalf("Received non-200 response: %d\n", resp.StatusCode)
		}
		expected := fmt.Sprintf("Visitor count: %d.", i)
		actual, err := ioutil.ReadAll(resp.Body)
		if err != nil {
			t.Fatal(err)
		}
		if expected != string(actual) {
			t.Errorf("Expected the message '%s'\n", expected)
		}
	}
}

Do you know how to test the main.go? I'm trying to reach 100% code coverage here.

100% code coverage is not a useful thing to aspire to. You'll end up writing strange code for testing, rather than for solving the problem you're trying to solve.