fix MKS937B check_set_errors functions and improve tests

[dkriegner]

previously the test did not cover any property setter. Also the check_set_errors function did not read back the additional termination characters. check_set_errors now returns a list upon success as required.

This fixes #935

[BenediktBurger]

Thanks for the fix. It looks good.
Did you test the change with a device, that the additional read does not break the device?

[dkriegner]

I checked it with the hardware and the extra terminators are sent by the device as mimicked by the test code. It previously went unnoticed since the extra terminator characters were ignored by the regular expression parsing the reply.

[BenediktBurger]

Thanks for checking.