lorder: Tweak invalid file test.

Different implementations of `nm` have different ways of telling you
that your file is not a valid object or library, but they all seem to
have “not recognized” as a common substring.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44154

lorder: Add unit tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44134