MM-41285: Skip flaky test TestAddChannelMemberFromThread (#19419)

Skipping the test for now. The proper fix will need
to be done by the team.

https://mattermost.atlassian.net/browse/MM-41285

```release-note
NONE
```

api4/channel_test.go

@@ -3013,6 +3013,7 @@ func TestAddChannelMember(t *testing.T) {
 }
 
 func TestAddChannelMemberFromThread(t *testing.T) {
+	t.Skip("MM-41285")
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 	team := th.BasicTeam