xfstests: fix NIS detection damage

NIS detection wasn't tested on machines without NIS enabled, so many tests are
failing on non-NIS machines. the _yp_active function has no specific return
value so always evaluates as 0 (active) and the "_cat_passwd" function is
called from within an awk script which is not valid as the shell may run with a
sanitised environment. Hence the functions do not need specific export calls,
either, as unsanitised subshells will automatically inherit the parent's
environment.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>