new: test: Mark each test's boundaries in the named instance logs

Use `rndc null` command to mark each test boundaries in named.run logs.

Since calling the `rndc` binary caused a lot of overhead, import the rndc python interface from [python-rndc](https://gitlab.isc.org/isc-projects/python-rndc/) and adapt it to our use-case.

Merge branch 'nicki/rndc-null-in-system-tests' into 'main'

See merge request isc-projects/bind9!12415