Resetting http handlers in golang for unit testing

I'm testing an http server in golang and everything seems pretty smooth except for one annoying thing.

At some point, when I'm configuring the server, before performing a http.ListenAndServe I register an handler with http.Handle("/", myrouter) , the problem is that on the following test, when the configuration method gets called again, I receive the following panic:

> panic: http: multiple registrations for / [recovered]

I'd like to run each test in a clean environment, but haven't found a way to tear down http.DefaultServeMux, is there a convenient way for doing this by either "nulling" something or re-configuring the test environment so every test is executed on a fresh environment ?

Edit:
To give some context, as asked, I post here some code and the test I wanted to write, I have to be clear tough on the fact that I'm not even sure on the implementation choices made here (I think I would have configured the server in a slightly different way, but the code's not mine).

package httpserver
import (
	"github.com/gorilla/mux"
	"github.com/private/private/httpserver/rpchandler"
	"net/http"
)
type HTTPServer struct {
	router *mux.Router
	port   int
}
// Config is used to override the default port of the http server.
type Config struct {
	Port *int
}
func NewHTTPServer(config *Config) (*HTTPServer, error) {
	hs := &HTTPServer{
		port: 80,
	}
	// Overwrite default port if passed.
	if config != nil && config.Port != nil {
		hs.port = *config.Port
	}
	err := hs.router = mux.NewRouter().StrictSlash(false)
	if err != nil {
		return nil, err
	}
    // other mux router configs here...
	http.Handle("/", hs.router)
	return hs, nil
}

Now, the tests I wanted to write were quite simple, like:

func TestThatServerIsInitializedWithDefaultPort(t *testing.T) {
	sut, _ := NewHTTPServer(nil)
	if sut.port != 80 {
		t.Fatal("default port not configured")
	}
}
func TestThatServerDefaultPortIsOverriddenWithConfig(t *testing.T) {
	mockPort := 8080
	c := Config{
		Port: &mockPort,
	}
	sut, _ := NewHTTPServer(&c)
	if sut.port != 8080 {
		t.Fatal("the port has not been overridden with the one passed in configuration")
	}
}

However, since I call the handle binding on http twice, I get the panic.

I think I've found a solution: basically after each test I reinitialize http.DefaultServeMux with http.DefaultServeMux = new(http.ServeMux)
I'm still not sure this is a clean way to workaround the problem tough.

I kindly ask you to give me any hints or point me some bad practices used here, since I'm quite new to the language and to backend development in general.
I think there might be some code smell in the configuration of the HTTP Server shown above, if you point it out I might make it clear to the rest of my team and work a better solution out.

I think you should look at using the httptest pkg. It has servers that you can start fresh every test.