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.