fix(p2p): fix race condition caused by startTestNetwork (#801)

The `require.NotEmpty` utility reads through the struct fields of the
Host via reflection, while the internal background routine in the host
also accesses its field, causing race conditions. The fix is to use
`require.NotNil` as it only checks whether the Host is nil or not
without accessing its internal fields

Author: Wondertan

p2p/utils_test.go

@@ -80,7 +80,7 @@ func startTestNetwork(ctx context.Context, t *testing.T, n int, conf map[int]hos
 			require.NoError(err)
 			host, err := mnet.AddPeer(privKey, addr)
 			require.NoError(err)
-			require.NotEmpty(host)
+			require.NotNil(host)
 		} else {
 			_, err := mnet.GenPeer()
 			require.NoError(err)