test-includes: use the same header set for badfiles.inc as for testing

This ensures we don't end up listing files we've excluded (e.g. those
starting with _).
Add a slight hack to preserve the existing order (sys, then net) in
badfiles.inc.

Reviewed by: imp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33505