pyln: failing test msat from float str

Currently we are not able to create pyln Millisatoshi from floats, e.g.:
 - "0.01btc"
 - "0.1sat"
 - ...

This adds a test that is currently expected to fail because of this bug.

Changelog-None

contrib/pyln-client/tests/test_units.py

@@ -1,4 +1,5 @@
 from pyln.client import Millisatoshi
+import pytest
 
 
 def test_to_approx_str():
@@ -34,3 +35,13 @@ def test_to_approx_str():
     assert amount.to_approx_str() == "12btc"
     amount = Millisatoshi('1200000000sat')
     assert amount.to_approx_str(1) == "12btc"  # note: no rounding
+
+
+@pytest.mark.xfail
+def test_floats():
+    amount = Millisatoshi("0.01btc")
+    assert amount.to_satoshi() == 10**6
+    amount = Millisatoshi("0.1sat")
+    assert int(amount) == 100
+    with pytest.raises(ValueError):
+        amount = Millisatoshi("0.1msat")