add expected: fail test for buffer comparison

test/tests/buffer_compare.py

+# expected: fail
+# we don't support tp_compare yet
+
+s = 'Hello world'
+t = buffer(s, 6, 5)
+
+s2 = "Goodbye world"
+t2 = buffer(s2, 8, 5)
+
+print t == t2