search frontend: fix parser adding extraneous node

Created by: rvantonder

Frontend parser added/duplicated a node when it reached the end of a group of terms. It turns out we already have a test case (with an incorrect oracle) for this, that I missed in the initial implementation. I added another one that's similar but different that I care about.

This doesn't affect any features that I'm aware of, since the parser output isn't yet used to drive parts of our logic.

@novoselrok ran into this while trying to use the parser, thanks for trialing :-)

I will reformat the tests in this file to be more readable in next PR.