chg: nil: Update the system test README for the pytest-native workflow

The README predated the meson migration and most of the pytest runner
features. Document building the test dependencies with meson and drop
the make-based instructions, the Makefile.am registration step, and the
stale -T flag list. Describe the jinja2 templating, bootstrap(), the
conftest fixtures, the pytest marks, and recommend node IDs and
parametrization over -k matching. Fix the directory naming rule, which
switched from hyphens to underscores.

Also declare pytest and pytest-xdist as required dependencies: the
runner's pytest.ini uses --dist=loadscope unconditionally, so pytest
without pytest-xdist cannot even start.

Related #3810

Assisted-by: Claude:claude-fable-5
Merge branch 'nicki/systest-readme-refresh' into 'main'

See merge request isc-projects/bind9!12232