.github: workflows: Add test trigger for pytest harness

Pytest twister harness tests have been added to the
twister_tests.yml workflow some time ago.

However, the $ZEPHYR_BASE/scripts/pylib/pytest-twister-harness/**
has not been monitored by the automatic GitHub Action triggers
of that workflow.

This change adds monitoring for the relevant files.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>