game: fix handling of useless SCCs

This is a bug I introduced while merging Philipp's patch.

* spot/misc/game.cc (parity_game::solve): Set the strategy for player
0 in useless SCCs.
(parity_game::fix_scc): Do not use sub_game_ to detect edges exiting
the SCC.
* tests/python/game.py: New file.
* tests/Makefile.am: Add it.

tests/python/game.py

@@ -0,0 +1,63 @@
+#!/usr/bin/python3
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2020 Laboratoire de Recherche et Développement de
+# l'EPITA.
+#
+# 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
+
+g = spot.automaton("""HOA: v1 States: 9 Start: 0 AP: 2 "a" "b"
+acc-name: Streett 1 Acceptance: 2 Fin(0) | Inf(1) properties:
+trans-labels explicit-labels state-acc spot-state-player: 0 1 0 1 0 1
+0 1 1 --BODY-- State: 0 {1} [1] 1 [1] 3 State: 1 {1} [1] 2 State: 2
+{1} [0] 8 State: 3 {1} [1] 4 State: 4 {1} [0] 5 State: 5 {1} [0] 6
+State: 6 {1} [1] 7 State: 7 State: 8 {1} [0] 2 --END--""")
+
+assert spot.solve_parity_game(g) == False
+
+s = spot.highlight_strategy(g).to_str("HOA", "1.1")
+assert s == """HOA: v1.1
+States: 9
+Start: 0
+AP: 2 "a" "b"
+acc-name: Streett 1
+Acceptance: 2 Fin(0) | Inf(1)
+properties: trans-labels explicit-labels state-acc !complete
+properties: !deterministic exist-branch
+spot.highlight.states: 0 5 1 4 2 4 3 5 4 5 5 5 6 5 7 5 8 4
+spot.highlight.edges: 2 5 3 4 6 5 8 5 9 4
+spot.state-player: 0 1 0 1 0 1 0 1 1
+--BODY--
+State: 0 {1}
+[1] 1
+[1] 3
+State: 1 {1}
+[1] 2
+State: 2 {1}
+[0] 8
+State: 3 {1}
+[1] 4
+State: 4 {1}
+[0] 5
+State: 5 {1}
+[0] 6
+State: 6 {1}
+[1] 7
+State: 7
+State: 8 {1}
+[0] 2
+--END--"""