Add better tests for special ($) query operators

[adewes]

Currently there are only two tests in tests/test_querying.py that cover various special ($) query operators.

Improve Blitz by adding more specialized tests for the various operators:

$in
$exists
$all
$regex
$ne
$eq
$not
$lt
$gt
$lte
$gte
$and
$or
#...

1. Create a new file tests/test_query_operators.py
2. Add appropriate tests for all operators listed above, possibly using existing test data or generating new test data, where appropriate.

[adewes]

@cBrauge is working on this