Also implement this in the dummy or it crashed

There is no way to write to read-only variables.
Nevertheless, add a check in the dummy to protect against that

The payload of the response frame was not set to the payload of the VS
frame

  - Implementation done
  - Revert lines for manual testing from testReadOnDemand.cc

Removes need for call to_upper() which would also cause trouble because
the CRC values would not match if the string is modified after
caluclating the CRC

Implements #6654

TecDummy::getLastWriteToFlash now provides information on if the
queried parameter was written to the flash memory previously.

Implements #6523

Implements #6524