Skip TestPreparePostForClient (#18100)

https://mattermost.atlassian.net/browse/MM-37757

```release-note
NONE
```

app/post_metadata_test.go

@@ -521,6 +521,7 @@ func TestPreparePostForClient(t *testing.T) {
 	})
 
 	t.Run("no metadata for deleted posts", func(t *testing.T) {
+		t.Skip("MM-37757")
 		th := setup(t)
 		defer th.TearDown()