chg: test: Reimplement the `formerr` system test in Python

Replace the shell and Perl based FORMERR system test with a Python
test that constructs the malformed DNS packets directly and checks
the responses.

Remove the legacy shell and Perl test script and the intermediate
packet files in hex, leaving the packet construction inline in
tests_formerr.py.

Merge branch 'stepan/formerr-system-test-python' into 'main'

See merge request isc-projects/bind9!11898