Properly join list of methods in _DynamicCapabilityClient

This was already fixed in c9bea05f44, but the fix does not seem to work.
This commit uses a set union, which should be more robust. It also adds
a couple of assertions to verify that it indeed works.

capnp/lib/capnp.pyx

@@ -2199,7 +2199,7 @@ cdef class _DynamicCapabilityClient:
             return self._cached_schema
 
     def __dir__(self):
-        return list(set(self.schema.method_names_inherited + tuple(dir(self.__class__))))
+        return list(set(self.schema.method_names_inherited) | set(dir(self.__class__)))
 
 
 cdef class _CapabilityClient: