frontend/parser_test: Cover more basic expressions.
parser: Make sure that each comparator matcher is a host matcher.

So far, 'NOT (service|attribute).<attr> <cmp> <value>' matches did not work
correctly, because the object matcher was not correctly wrapped into a host
matcher. Since we do not currently support querying any object types besides
hosts, this was wrong.

The respective code has been moved into sdb_store_matcher_parse_cmp() which
also implements and checks the restriction on matching host objects only.

A new set of tests has been introduced which exposed this problem and which
(roughly) verifies the result of a lookup using a parsed matcher.
store_lookup_test: Fixed user-data passed to the lookup callback.
store_lookup: Added "tostring" methods for matcher objects.

This is meant for logging and debugging. For now, it's used in tests to check
parsed and compiled matchers.