Fix most memory leaks recently introduced
- Do not overwrite memory allocated in init_mtop() when creating
molecules in selection tests.
- Add function to properly deallocate t_topology and gmx_mtop_t when
gmx_mtop_t_to_t_topology(..., false) has been called.
- Free all memory in done_mtop().
These should be enough to (again) make the unit tests free of memory
leaks, except that t_state.enerhist is leaked in a few places. That
requires more complicated handling.
Change-Id: I84b853b9cb8a11c0458ea4f29316a83a41d7ae22