Extend automated string tests to cover the formerly-crashing "[]" substitution...

Extend automated string tests to cover the formerly-crashing "[]" substitution case (EXT-1282, DEV-41081)

Also 'fix' string tests to match the new expected behaviour of LLStringUtil::format().  They must have been failing dreadfully before, but the legacy tut tests aren't run in a viewer-only build, so...