zlktree: fix a bug

Reported by Florian.

* spot/twaalgos/zlktree.cc: Handle the case where the condition does
not cover all colors.
* tests/python/zlktree.py: New file.
* tests/Makefile.am: Add it.

tests/Makefile.am

@@ -450,6 +450,7 @@ TESTS_python = \
   python/twagraph.py \
   python/toweak.py \
   python/_word.ipynb \
+  python/zlktree.py \
   $(TESTS_ipython)
 endif
 

tests/python/zlktree.py

@@ -0,0 +1,29 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2021 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/>.
+
+import spot
+
+a = spot.automaton("""HOA: v1 States: 5 Start: 0 AP: 2 "p0" "p1"
+Acceptance: 4 Inf(3) | Fin(3) properties: trans-labels explicit-labels
+trans-acc --BODY-- State: 0 [!0&!1] 3 [!0&!1] 4 State: 1 [!0&!1] 4 {3}
+[0&!1] 0 {2} [!0&1] 1 {2} State: 2 [!0&1] 0 {0 2} [!0&!1] 1 State: 3
+[!0&1] 2 State: 4 [0&!1] 3 --END--""")
+b = spot.zielonka_tree_transform(a)
+assert spot.are_equivalent(a, b)
+assert b.acc().is_buchi()