Add `open_root_actor(ensure_registry: bool)`

Allows forcing the opened actor to either obtain the passed registry
addrs or raise a runtime error.

tractor/_root.py

@@ -85,6 +85,10 @@ async def open_root_actor(
     enable_modules: list | None = None,
     rpc_module_paths: list | None = None,
 
+    # NOTE: allow caller to ensure that only one registry exists
+    # and that this call creates it.
+    ensure_registry: bool = False,
+
 ) -> Actor:
     '''
     Runtime init entry point for ``tractor``.
@@ -206,6 +210,12 @@ async def open_root_actor(
     # REGISTRAR
     if ponged_addrs:
 
+        if ensure_registry:
+            raise RuntimeError(
+                 f'Failed to open `{name}`@{ponged_addrs}: '
+                'registry socket(s) already bound'
+            )
+
         # we were able to connect to an arbiter
         logger.info(
             f'Registry(s) seem(s) to exist @ {ponged_addrs}'