p2p/test: wait for listener to get ready (#4881)

Attempt to fix this the below test failure by waiting for the listener to get ready. I am not at all convinced that this is the correct fix - the below indicates that the TCP socket was closed after it was set up - but I'm unable to come up with an actionable hypothesis for what caused it.

```
2020/05/14 17:25:11 Failed to accept conn: accept tcp 127.0.0.1:44737: use of closed network connection
2020/05/14 17:25:11 Failed to accept conn: accept tcp 127.0.0.1:42589: use of closed network connection
2020/05/14 17:25:11 Failed to accept conn: accept tcp 127.0.0.1:40905: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:39847: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:39989: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:43587: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:35415: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:38657: use of closed network connection
2020/05/14 17:25:12 Failed to accept conn: accept tcp 127.0.0.1:38217: use of closed network connection
2020/05/14 17:25:13 Failed to accept conn: accept tcp 127.0.0.1:42247: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:39705: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:39491: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:37107: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:39909: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:37987: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:41505: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:39121: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:46569: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:45643: use of closed network connection
2020/05/14 17:25:16 Failed to accept conn: accept tcp 127.0.0.1:35289: use of closed network connection
--- FAIL: TestTransportMultiplexAcceptMultiple (0.43s)
    transport_test.go:200: auth failure: handshake failed: EOF
FAIL
```

p2p/transport_test.go

@@ -633,6 +633,9 @@ func testSetupMultiplexTransport(t *testing.T) *MultiplexTransport {
 		t.Fatal(err)
 	}
 
+	// give the listener some time to get ready
+	time.Sleep(20 * time.Millisecond)
+
 	return mt
 }