Create a helper to find a free TCP port

[0x501D]

In integration tests we have hardcoded ports:
Example

This can cause tests to fail:

    def test_play_test_remote_instance(tt_cmd, tmpdir):
        # Copy the .xlog and instance config files to the "run" directory.
        test_app_path = os.path.join(os.path.dirname(__file__), "test_file")
        shutil.copy(test_app_path + "/test.xlog", tmpdir)
        shutil.copy(test_app_path + "/remote_instance_cfg.lua", tmpdir)
    
        # Play .xlog file to the remote instance.
        cmd = [tt_cmd, "play", "localhost:3301", "test.xlog", "--space=999"]
        instance = subprocess.Popen(["tarantool", "remote_instance_cfg.lua"], cwd=tmpdir)
        # The delay is needed so that the instance has time to start and configure itself
        sleep(1)
        rc, output = run_command_and_get_output(cmd, cwd=tmpdir)
        instance.kill()
>       assert rc == 0
E       assert 1 == 0

test/integration/play/test_play.py:51: AssertionError

It is proposed to create a helper that looks for a free TCP port. This will greatly reduce the chance of falsely failed tests.

[Totktonada]

We have the long standing issue (races) due to the check-free-port-and-use-it approach in test-run, see tarantool/test-run#141. I suggest to at least try to implement binding to a zero port.