Fix BStoBS+Reco test and extend rewriteLVL1 option to work in offline athena

Three separate changes but all needed to make the BS->BS (offline athena) -> ESD->AOD path work. This makes the trigAna_BStoBStoESDAOD test finally work and produce the expected outputs. The changes are:

1. Fix pre-exec options in test_trigAna_BStoBStoESDAOD_mt1_build.py (fixes ATR-21648 and further downstream issues)
2. Add offline-athena support to the rewriteLVL1 modifier
3. Move the modifiers' postSetup call in runHLT_standalone.py to the end of the file (more correct and required for 2.)