search: query_approxidate: cleanup regexp, more tests

search: query_approxidate: cleanup regexp, more tests

The cleanup doesn't seem to matter, I initially thought I needed
to handle "" (two double quotes) explicitly because that's what
Xapian does to escape a double quote inside a double-quoted
phrase.  It turns out we only need to be able to pass phrases
through to Xapian unmodified, and the existing group of
["\x{201c}\x{201d}] is sufficient for our purposes.