Get createSaveTestPeaksWorkspace to use buildPW re #7674

I found out that workspace and instrument created by buildPW was adequate for my testing requirements.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Improve comment of BuildPW re #7674

The comment now indicates that the workspace has one peak. This peak delayed work till I was aware of it.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>