pylightning: adds a millisatoshi test class

contrib/pylightning/tests/test_millisatoshi.py

@@ -0,0 +1,6 @@
+from lightning import Millisatoshi
+
+
+def test_sum_radd():
+    result = sum([Millisatoshi(1), Millisatoshi(2), Millisatoshi(3)])
+    assert int(result) == 6