[9.18] new: test: Add module-specific python setup to system tests

During the system test execution, allow use of module-specific setup()
function in addition to the setup.sh script which this function should
ultimately replace.

The purpose of setup() is two-fold. First, it can execute any commands
needed to create the initial conditions for the test, such as creating
key materials, manipulating files etc. Second, it should return any
test-specific template values as a dictionary. Those will be used to
render the jinja2 templates.

Backport of MR !10983

Merge branch 'backport-nicki/pytest-add-python-setup-func-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11127