tests: Reserve "Internal" client, used by email gateway and topic moves.

(cherry picked from commit 13b9c87f93)

zerver/lib/server_initialization.py

@@ -53,6 +53,7 @@ def create_internal_realm() -> None:
     # just ensures these get low IDs in production, and in development
     # avoids an extra database write for the first HTTP request in
     # most tests.
+    get_client("Internal")
     get_client("website")
     get_client("ZulipMobile")
     get_client("ZulipElectron")