pytest: check millisatoshi by float raises

This only adds a test that currently makes sure its not possible
to init a Millisatoshi by a floating number.

Discussion:
As @gallizoltan points out, initialization with a float should be possible:
https://github.com/ElementsProject/lightning/pull/4273#discussion_r540369093

> Millisatoshi(5) / 2 currently works, and removes the half msat.
> So, I think Millisatoshi(5 / 2) should be the same.

contrib/pyln-client/pyln/client/lightning.py

@@ -68,6 +68,10 @@ class Millisatoshi:
 
         elif int(v) == v:
             self.millisatoshis = int(v)
+
+        elif isinstance(v, float):
+            raise TypeError("Millisatoshi by float is currently not supported")
+
         else:
             raise TypeError(
                 "Millisatoshi must be string with msat/sat/btc suffix or int"