PLCgen TwinCAT self-certification: generated solution files are not valid

Test report

Tool	Version
ESCET	nightly-20241023-115319
TwinCat	v3.1 build 4020.56
Test suite	v27

Setup of TwinCAT as described here: https://cstweb.wtb.tue.nl/4tc00/twincat/setup.html#twincat-install.

Encountered errors

All generated .snl solution files are invalid: error when opening the file in TwinCAT The selected file is not a valid solution file.

Addresses #1020 (closed) #679