tests: introduce more test coverage of whoosh filtering This has been extracted from other changesets by Mads Kiilerich to establish a test baseline so we clearly can see what the following fixes are fixing. Some of these tests will thus demonstrate bad behaviour - that will be fixed later.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.