Updating FastChain ART tests

Small fix in config flags in FastChain legacy-based skeleton

All FastChain ART tests were updated:

• MC20a tests deleted (due to failures and obsolescence - they can be added after they are fixed)
• Updates made to MC21a tests
  • NTUP step fixed
  • Legacy-based tests deleted
  • New tests running Hybrid-overlay with ML-based decision (whether an event will be reconstructed by MC- or Track-overlay) added
    • Hybrid-overlap test (test_MC21a_ATLFAST3F_G4MS_HybridOverlay_ttbar_CA.sh): with a predefined threshold of 0.74201
    • MC-overlay 2 test (test_MC21a_ATLFAST3F_G4MS_MCOverlay2_ttbar_CA.sh): with the threshold set to 0.1 (should provide the same physics results as the test with MC-overlap)
    • Track-overlap test 2 (test_MC21a_ATLFAST3F_G4MS_TrackOverlay2_ttbar_CA.sh): with the threshold set to 0.95 (should provide the same physics results as the test with Track-overlap)

cc @jchapman, @tadej