plugin: Allow custom features only if the plugin is not dynamic

This is in order to avoid having to update featurebits as plugins get
activated and deactivated.

tests/plugins/feature-test.py

@@ -5,6 +5,7 @@ from pyln.client import Plugin
 # Register a different set feature of feature bits for each location so we can
 # later check that they are being passed correctly.
 plugin = Plugin(
+    dynamic=False,
     init_features=1 << 101,
     node_features=1 << 103,
     invoice_features=1 << 105,