Flaky test due to inter-process race on finding a free port

[annagrin]

See: https://github.com/dart-lang/webdev/runs/5251701775?check_suite_focus=true

05:17 +208 ~2 -1: test/run_request_test.dart: while debugger is attached can resume while paused at the start [E]                                                                                      
  SocketException: Failed to create server socket (OS Error: Address already in use, errno = 98), address = 127.0.0.1, port = 40583
  package:http_multi_server/http_multi_server.dart 209:20  HttpMultiServer._loopback
  ===== asynchronous gap ===========================
  test/fixtures/server.dart 126:18                         TestServer.start
  ===== asynchronous gap ===========================
  test/fixtures/context.dart 302:20                        TestContext.setUp
  ===== asynchronous gap ===========================
  test/run_request_test.dart 25:7                          main.<fn>.<fn>