Draggable: fix changing containment

If containment was set such that relative_container is set by
_setContainment, and then containment changes to e.g. "document",
"window", or an array, relative_container would not be unset, causing
incorrect containment of the draggable.

Add a unittest to check that containment with an array works after
previously being set to "parent".

Fixes #9733