Add test for Xerror property to SaveAsciiTest.h re #7645

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Remove unused variable re #7645

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge remote-tracking branch 'origin/feature/7645_saveascii_writeXerror'