Guile2: fix two tests in report system

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23441 57a11ea4-9604-0410-9ed3-97b8803252fd
Preferences fixes

- move GNC_GSETTINGS_PREFIX parsing to a location that it is also used in the python bindings
- read backup retain policy related settings before testing if they are valid

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23442 57a11ea4-9604-0410-9ed3-97b8803252fd
Bug 712299 - Tax Invoice with guile 2 doesn't display currency symbols (second attempt)

This re-applies commit r23416 and fixes the test reports by
setting a fixed locale before running the tests. This should
ensure a predictable report output to compare against

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23443 57a11ea4-9604-0410-9ed3-97b8803252fd