Better tests directory structure (#2231)

tests/utils/find_free_port.py

@@ -0,0 +1,12 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+import socket
+from contextlib import closing
+
+
+def find_free_port() -> str:
+    """Returns the available host port. Can be called in multiple threads/processes."""
+    with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
+        s.bind(("", 0))
+        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        return s.getsockname()[1]  # type: ignore