[9.18] new: test: add helper functions to isctest

added some helper functions in isctest to reduce code repetition
in dnssec-related tests:

- isctest.check.adflag() - checks that a response contains AD=1
- isctest.check.noadflag() - checks that a response contains AD=0

- isctest.check.rdflag() - checks that a response contains RD=1
- isctest.check.nordflag() - checks that a response contains RD=0

- isctest.check.raflag() - checks that a response contains RA=1
- isctest.check.noraflag() - checks that a response contains RA=0

- isctest.check.rr_count_eq() - checks the number of RRsset in a section

- isctest.check.same_data() - checks that two message have the
                              same rcode and data
- isctest.check.same_answer() - checks that two message have the same
                                rcode and answer

- isctest.query.create() - a wrapper for dns.message.make_query() that
                           creates a query message similar to dig +dnssec

Backport of MR !10760

Merge branch 'backport-each-isctest-helpers-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10794