Pagination tests fail on travis with postgresql

The problem is the posts are generated too fast (multiple posts per second), since the container speed up (we had this issue previously aswell).

This causes reply 9 and 10 randomly to be sorted incorrectly (but if it happens once, it will always happen).
I suggest instead of checking for post 9 and 11, we check 4 and 16.
This way enough time has passed by, so the posts are sorted onto the right page.