pad: Fix problem with destroy callback not being called

pad: Fix problem with destroy callback not being called

When we unblock a pad with the same user_data, the destroy callback is not
called. This leads to refcounting leaks that cannot be avoided. Instead always
call the destroy notify whenever we install a new pad block.
In particular, this fixes a nasty pad leak in decodebin2.

Also update the unit test to have more accurate comments and test the required
behaviour.