Added some tests for some `ClientModel` bits which were not covered.
Added some tests for some `ClientModel` bits which were not covered.

Specifically, `ClientModel::move_window`,
`ClientModel::resize_window`, `ClientModel::focus` and
`ClientModel::unfocus` were not tested previously but now have
corresponding tests (which pass).

8l/swm,8l/SmallWM,8l/swm,8l/SmallWM