Fix usr.sbin/extattr testcases on tmpfs

Skip the usr.sbin/extattr testscases if $TMPDIR is tmpfs, which doesn't
support extended attributes

PR: 210184
Reported by: ngie
Reviewed by: ngie
Approved by: re (glebius)
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D6802
Sponsored by: Spectra Logic Corp