tests: fix wait_tcp_port format string.

The success-case `print` for the `wait_tcp_port` helper wasn't
formatting its message as an f-string, instead printing the literal
`{host}` and `{port}` placeholders.

tests/client-server.py

@@ -32,7 +32,7 @@ def wait_tcp_port(host, port):
     else:
         print("client-server.py: unable to connect")
         sys.exit(1)
-    print("client-server.py: detected server is up on {host}:{port}")
+    print(f"client-server.py: detected server is up on {host}:{port}")
 
 class Failure(Exception):
     pass