determinize: don't emit colors for temporary braces

Related to issue #298.

* spot/twaalgos/determinize.cc: Recognize braces that are temporary
to avoid emitting colors when they become empty.
* tests/python/298.py: New file, showing a reduction of colors.
* tests/Makefile.am: Add it.
* tests/core/ltlsynt.test: Adjust expected output (now smaller).
* tests/core/genltl.test: Adjust one expected output (now larger).
* NEWS: Mention the issue.

tests/python/298.py

@@ -0,0 +1,33 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2020 Laboratoire de Recherche et Développement de l'Epita
+# (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test for parts of Issue #298.
+
+import spot
+
+a1 = spot.automaton("""genltl --dac=51 | ltl2tgba --med |""")
+a1 = spot.degeneralize_tba(a1)
+r1 = spot.tgba_determinize(a1, True, False, False)
+assert r1.num_sets() == 3
+
+a2 = spot.automaton("""genltl --dac=51 | ltl2tgba --high |""")
+a2 = spot.degeneralize_tba(a2)
+r2 = spot.tgba_determinize(a2, True, False, False)
+# This used to fail in 2.9.5 and earlier.
+assert r2.num_sets() == 3