Fix LoadHTML* tests (#1559)

Digging into the test code base I've found out that some of the tests for `LoadHTML*` methods are not reliable and efficient. They use timeouts to be sure that goroutine with the server has started. And even more, in old implementation, the server started only once – all the new instances silently failed due to the occupied network port.

Here is a short overview of the proposed changes: 
- it's not necessary to rely on timeouts, the server starts listening synchronously and returns control when it is ready
- once the server is run, it's stopped after a test passes
- dry out http server setup
- magic with empty closure return is eliminated 
- preserve router.RunTLS coverage with integration tests

gin_integration_test.go

@@ -6,6 +6,7 @@ package gin
 
 import (
 	"bufio"
+	"crypto/tls"
 	"fmt"
 	"io/ioutil"
 	"net"
@@ -20,7 +21,14 @@ import (
 )
 
 func testRequest(t *testing.T, url string) {
-	resp, err := http.Get(url)
+	tr := &http.Transport{
+		TLSClientConfig: &tls.Config{
+			InsecureSkipVerify: true,
+		},
+	}
+	client := &http.Client{Transport: tr}
+
+	resp, err := client.Get(url)
 	assert.NoError(t, err)
 	defer resp.Body.Close()
 
@@ -45,6 +53,22 @@ func TestRunEmpty(t *testing.T) {
 	testRequest(t, "http://localhost:8080/example")
 }
 
+func TestRunTLS(t *testing.T) {
+	router := New()
+	go func() {
+		router.GET("/example", func(c *Context) { c.String(http.StatusOK, "it worked") })
+
+		assert.NoError(t, router.RunTLS(":8443", "./testdata/certificate/cert.pem", "./testdata/certificate/key.pem"))
+	}()
+
+	// have to wait for the goroutine to start and run the server
+	// otherwise the main thread will complete
+	time.Sleep(5 * time.Millisecond)
+
+	assert.Error(t, router.RunTLS(":8443", "./testdata/certificate/cert.pem", "./testdata/certificate/key.pem"))
+	testRequest(t, "https://localhost:8443/example")
+}
+
 func TestRunEmptyWithEnv(t *testing.T) {
 	os.Setenv("PORT", "3123")
 	router := New()