Add browser test for adding a grammatical features.

This adds a test for entering and selecting an item in the grammatical
feature widget and checks whether the selected item is displayed as a
grammatical feature afterwards.

Additional dependencies to the Wikibase browser tests had to be added in
order to make use of item creation steps.

Bug: T163972
Change-Id: Iee8b62103587ab16e55eb10a5982e3b1b978d95b