tests: Fixed flaky test by using user joined callback. (shazow#393)

Instead of relying on the go scheduler to do the expected thing >_>

Co-authored-by: Akshay <akshay.shekher@gmail.com>

host_test.go

@@ -78,8 +78,6 @@ func TestHostGetPrompt(t *testing.T) {
 }
 
 func TestHostNameCollision(t *testing.T) {
-	t.Skip("Test is flakey on CI. :(")
-
 	key, err := sshd.NewRandomSigner(512)
 	if err != nil {
 		t.Fatal(err)
@@ -93,9 +91,13 @@ func TestHostNameCollision(t *testing.T) {
 	}
 	defer s.Close()
 	host := NewHost(s, nil)
+
+	newUsers := make(chan *message.User)
+	host.OnUserJoined = func(u *message.User) {
+		newUsers <- u
+	}
 	go host.Serve()
 
-	ready := make(chan struct{})
 	g := errgroup.Group{}
 
 	// First client
@@ -111,8 +113,8 @@ func TestHostNameCollision(t *testing.T) {
 				t.Errorf("Got %q; expected %q", actual, expected)
 			}
 
-			// Ready for second client
-			ready <- struct{}{}
+			// wait for the second client
+			<-newUsers
 
 			scanner.Scan()
 			actual = scanner.Text()
@@ -127,20 +129,16 @@ func TestHostNameCollision(t *testing.T) {
 				t.Errorf("Got %q; expected %q", actual, expected)
 			}
 
-			// Wrap it up.
-			close(ready)
 			return nil
 		})
 	})
 
-	// Wait for first client
-	<-ready
-
 	// Second client
 	g.Go(func() error {
+		// wait for the first client
+		<-newUsers
 		return sshd.ConnectShell(s.Addr().String(), "foo", func(r io.Reader, w io.WriteCloser) error {
 			scanner := bufio.NewScanner(r)
-
 			// Consume the initial buffer
 			scanner.Scan()
 			scanner.Scan()