Make SaveGSASInstrumentFileTest.h use full pathname re #8073

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Also make the change to a disabled test re #8073

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Hopefully fix an illegal string operation tolerate by Windows re #8073

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Fix a compiler warning re #8073

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge remote-tracking branch 'origin/feature/8073_check_file_existence'