Added a basic unit test for strategy (needs to be extended).

tests/test_strategy.py

+from src.rules.factors import match_expand, expand_double, expand_single
+from src.node import Scope
+from src.possibilities import Possibility as P
+from src.strategy import find_possibilities
+from tests.rulestestcase import RulesTestCase, tree
+
+
+class TestStrategy(RulesTestCase):
+
+    def test_find_possibilities_sort(self):
+        (ab, cd), e = root = tree('(a + b)(c + d)e')
+        self.assertEqualPos(find_possibilities(root),
+                [P(root, expand_single, (Scope(root), cd, e)),
+                 P(root, expand_single, (Scope(root), ab, e)),
+                 P(root, expand_double, (Scope(root), ab, cd))])