pytest: test that we translate to and from Millisatoshi on plugin RPC.

We don't, but we should, like we do for normal RPC.  However, I chose
to use function annotations, rather than names-ending-in-'msat'
because it's more Pythony.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/millisatoshis.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+from lightning import Plugin, Millisatoshi
+
+
+plugin = Plugin(autopatch=True)
+
+
+@plugin.method("echo")
+def echo(plugin, msat: Millisatoshi, not_an_msat):
+    plugin.log("got echo for {} {} (types {} and {})"
+               .format(msat, not_an_msat, type(msat), type(not_an_msat)))
+    if not isinstance(msat, Millisatoshi):
+        raise TypeError("msat must be Millisatoshi not {}".format(type(msat)))
+    if isinstance(not_an_msat, Millisatoshi):
+        raise TypeError("not_an_msat must not be Millisatoshi")
+    plugin.log("got echo for {} (type {})".format(msat, type(msat)))
+    if not isinstance(msat, Millisatoshi):
+        raise TypeError("msat must be Millisatoshi not {}".format(type(msat)))
+    plugin.log("Returning {}".format(msat))
+    return {'echo_msat': msat}
+
+
+plugin.run()