test/cli-plugins: Try to make TestConnectAndWait less flaky

- Add runtime.Gosched() calls to encourage goroutine scheduling
- Increase the timeout from 10ms to 500ms
- Use poll.WaitOn with appropriate delays to ensure the goroutine has
  spawned before checking
- Lock the test goroutines to its own thread

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli-plugins/socket/socket_test.go

@@ -178,24 +178,39 @@ func TestConnectAndWait(t *testing.T) {
 	// TODO: this test cannot be executed with `t.Parallel()`, due to
 	// relying on goroutine numbers to ensure correct behaviour
 	t.Run("connect goroutine exits after EOF", func(t *testing.T) {
+		runtime.LockOSThread()
+		defer runtime.UnlockOSThread()
 		srv, err := NewPluginServer(nil)
 		assert.NilError(t, err, "failed to setup server")
 
 		defer srv.Close()
 
 		t.Setenv(EnvKey, srv.Addr().String())
+
+		runtime.Gosched()
 		numGoroutines := runtime.NumGoroutine()
 
 		ConnectAndWait(func() {})
-		assert.Equal(t, runtime.NumGoroutine(), numGoroutines+1)
+
+		runtime.Gosched()
+		poll.WaitOn(t, func(t poll.LogT) poll.Result {
+			// +1 goroutine for the poll.WaitOn
+			// +1 goroutine for the connect goroutine
+			if runtime.NumGoroutine() < numGoroutines+1+1 {
+				return poll.Continue("waiting for connect goroutine to spawn")
+			}
+			return poll.Success()
+		}, poll.WithDelay(1*time.Millisecond), poll.WithTimeout(500*time.Millisecond))
 
 		srv.Close()
 
+		runtime.Gosched()
 		poll.WaitOn(t, func(t poll.LogT) poll.Result {
+			// +1 goroutine for the poll.WaitOn
 			if runtime.NumGoroutine() > numGoroutines+1 {
 				return poll.Continue("waiting for connect goroutine to exit")
 			}
 			return poll.Success()
-		}, poll.WithDelay(1*time.Millisecond), poll.WithTimeout(10*time.Millisecond))
+		}, poll.WithDelay(1*time.Millisecond), poll.WithTimeout(500*time.Millisecond))
 	})
 }