prune_scc() leaked memory and failed to remove chains of useless SCCs.

* src/tgbaalgos/reductgba_sim.cc (reduc_tgba_sim): Call
scc_filter() instead of prune_scc(), and do it before running
any simulation-based reduction.
* src/tgbaalgos/reductgba_sim.hh (reduc_tgba_sim): Return a const
tgba*.
* src/tgbatest/ltl2tgba.cc: Call scc_filter() instead of
prune_scc().
* src/tgbatest/scc.test: Add two more tests that failed with
prune_scc().