message: Fix sign-compare warning-as-error in comms_test.
diff: @@ -71,7 +71,7 @@ TEST(CommTests, CommandRx) {
  pb_ostream_t stream = pb_ostream_from_buffer(
  reinterpret_cast<unsigned char *>(rx_buffer), sizeof(rx_buffer));
  pb_encode(&stream, Packet_fields, &packet);
  - EXPECT_GT(stream.bytes_written, 0);
  + EXPECT_GT(stream.bytes_written, 0u);
  Hal.test_serialPutIncomingData(rx_buffer, stream.bytes_written);
  EXPECT_GT(Hal.serialBytesAvailableForRead(), 0);
  