In anticipation of the changes in
I30aa36a0cb89988d1bad8256e5d3f737380bc0a7 which removes the
.wikibase-editor selector from the TalkSectionAddOverlay textarea, the
browser tests need to be changed to a more accomodating selector.
Instead of finding the textarea from the .wikitext-editor selector, the
tests now look for the textarea in the overlay (there is only one). This
also makes it more in line with its method for finding the input element
(.talk-overlay input) as well.
Bug: T221624
Change-Id: Ifbba23e9aee6f68033efc3279ed737d519a7bf41
723deeb8bf