Rules: fixed HandlesEntry for XMLSrc replacement

This makes HandlesEntry work with the new code that's in place for
StructFile to replace XMLSrc.  Fixes
https://github.com/Bcfg2/bcfg2/commit/2169edc1bba82076db776b75db89b79d6f2f4786#commitcomment-3968162
testsuite: rewrote Rules/Defaults tests to be actually useful