Listen explicitly on localhost on heartbeat TCP tests (elastic#15583)

Some tests were requiring localhost to resolve to 127.0.0.1, but this is
not always the case. In some machines localhost resolve to other local
addresses as 127.0.1.1, or to [::1] if ipv4 is not available.

NOTICE.txt

@@ -1,5 +1,5 @@
 Elastic Beats
-Copyright 2014-2019 Elasticsearch BV
+Copyright 2014-2020 Elasticsearch BV
 
 This product includes software developed by The Apache Software 
 Foundation (http://www.apache.org/).

heartbeat/monitors/active/tcp/tcp_test.go

@@ -103,22 +103,43 @@ func setupServer(t *testing.T, serverCreator func(http.Handler) *httptest.Server
  return server, port
 }
 
+// newLocalhostTestServer starts a server listening on the IP resolved from `localhost`
+// httptest.NewServer() binds explicitly on 127.0.0.1 (or [::1] if ipv4 is not available).
+// The IP resolved from `localhost` can be a different one, like 127.0.1.1.
+func newLocalhostTestServer(handler http.Handler) *httptest.Server {
+ listener, err := net.Listen("tcp", "localhost:0")
+ if err != nil {
+ panic("failed to listen on localhost: " + err.Error())
+ }
+
+ server := &httptest.Server{
+ Listener: listener,
+ Config: &http.Server{Handler: handler},
+ }
+ server.Start()
+
+ return server
+}
+
 func TestUpEndpointJob(t *testing.T) {
- server, port := setupServer(t, httptest.NewServer)
+ server, port := setupServer(t, newLocalhostTestServer)
  defer server.Close()
 
+ serverURL, err := url.Parse(server.URL)
+ require.NoError(t, err)
+
  event := testTCPCheck(t, "localhost", port)
 
  testslike.Test(
  t,
  lookslike.Strict(lookslike.Compose(
- hbtest.BaseChecks("127.0.0.1", "up", "tcp"),
+ hbtest.BaseChecks(serverURL.Hostname(), "up", "tcp"),
  hbtest.SummaryChecks(1, 0),
  hbtest.SimpleURLChecks(t, "tcp", "localhost", port),
  hbtest.RespondingTCPChecks(),
  lookslike.MustCompile(map[string]interface{}{
  "resolve": map[string]interface{}{
- "ip": "127.0.0.1",
+ "ip": serverURL.Hostname(),
  "rtt.us": isdef.IsDuration,
  },
  }),