added test for f_neg_canonical form

stl/test_utils.py

@@ -0,0 +1,13 @@
+import stl
+from stl.hypothesis import SignalTemporalLogicStrategy
+
+from hypothesis import given
+
+
+@given(SignalTemporalLogicStrategy)
+def test_f_neg_or_canonical_form(phi):
+    phi2 = stl.utils.f_neg_or_canonical_form(phi)
+    phi3 = stl.utils.f_neg_or_canonical_form(phi2)
+    assert phi2 == phi3
+    assert not any(
+        isinstance(x, (stl.ast.G, stl.ast.And)) for x in phi2.walk())