String improvements: For singular form of ngettest, the %d conversion specifier can be skipped.

See e.g. this thread http://article.gmane.org/gmane.comp.version-control.git/221583

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22914 57a11ea4-9604-0410-9ed3-97b8803252fd