This fixes the incorrect output of tgba_determinize() reported yesterday
by Reuben Rowe.
* spot/twaalgos/simulation.cc: Do not purge unreachable states when
recording implications.
* tests/python/bugdet.py: New test case.
* tests/Makefile.am: Add it.
* THANKS: Add Reuben.
* NEWS: Mention the bug.
