Give the autocomplete dropdown a signal tests can wait on (#2542)

Autocomplete dropdown items are rebuilt asynchronously, and a stale list
looks like a current one, so waiting for an item to be present waits for
nothing. Autocomplete now sets data-ac-search-text on the menu, naming
the search text its rendered items are for.

Adds gu.autocomplete (wait/getOptions/close), used in ReferenceColumns,
DropdownConditionEditor and ChoiceList. Three assertions in
DropdownConditionEditor expect an empty list, and passed whenever the
dropdown had yet to open.