Make the control server's connection-limit test pass on macOS

`cargo xtask ci` was red on a clean main on macOS: the test's last shutdown
runs after the server has answered an invalid request and closed the
connection, and `shutdown(2)` on a socket whose peer has already closed is
ENOTCONN on macOS and the BSDs where Linux returns success.

The test is right about what it checks; it checked a return value that is not
portable. Closing the client is what the worker thread waits for, and dropping
the socket does that whether or not shutdown reported anything. The file's
other six shutdowns run while the peer is still open, which is well defined on
both platforms, so they stay checked.

With no CI running the tests on pull requests, the local gate is the only
signal a contributor gets, and it could not be green on macOS.

apps/figcraft/src/control_server_tests.rs

@@ -160,7 +160,11 @@ fn connection_limit_rejects_excess_clients_and_reuses_finished_slots() {
     assert_eq!(workers.len(), 1);
     client.write_all(b"invalid\n").unwrap();
     assert_eq!(reply(&mut BufReader::new(client.try_clone().unwrap()))["ok"], false);
-    client.shutdown(Shutdown::Both).unwrap();
+    // The server closes this connection after answering an invalid request, and shutting down
+    // a socket whose peer has already closed is ENOTCONN on macOS and the BSDs where Linux
+    // returns Ok. Closing the client is what the worker is waiting for, so the result is moot:
+    // the other shutdowns in this file run while the peer is still open and stay checked.
+    let _ = client.shutdown(Shutdown::Both);
     workers.pop().unwrap().join().unwrap();
 }