MM-43252: Skip TestPreparePostForClient (#19959)

https://mattermost.atlassian.net/browse/MM-43252

```release-note
NONE
```

app/post_metadata_test.go

@@ -60,6 +60,7 @@ func TestPreparePostListForClient(t *testing.T) {
 }
 
 func TestPreparePostForClient(t *testing.T) {
+	t.Skip("MM-43252")
 	var serverURL string
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {