Add more pile-up configurations for Phase-II and update ART tests

Add more pile-up configurations for Phase-II:

• mu = 1 (actually 1 and 2)
• MC21a profile

Update ART tests for easier local running:

• use ATLAS_REFERENCE_DATA if available
• do not use art.py if not available

/cc @jchapman @bwynne @npetters @tstreble