newfs_msdos: connect the ATF test from NetBSD

NetBSD has an ATF test for newfs_msdos. Connect it to the build.
Adapt it for FreeBSD. This would have caught the bug fixed by my
previous commit.

Reviewed by: delphij, emaste
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34116