The goal is to improve coverage stats, but I discovered two issues
while doing so.
* tests/python/twagraph.py: New test case.
* tests/Makefile.am: Add it.
* spot/twa/twagraph.hh: Add fix typos in error messages.
* python/spot/impl.i: Fix broken wrappers for state_from_number and
state_acc_sets.
