makefs: tests: Fix -D flag test

As stated in the manual page:

-F is almost certainly not the option you are looking for. To
create an image from a list of files in an mtree format manifest,
specify it as the last argument on the command line, not as the
argument to -F.

This change does exactly that. Also bug #192839 has already been fixed.

Reviewed by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54428