fstests: generic test for listing an inode's xattrs

fstests: generic test for listing an inode's xattrs

Test that the filesystem's implementation of the listxattrs system call
lists all the xattrs an inode has.

This test is motivated by a bug found in btrfs, which is fixed by the
following patch for the linux kernel:

  "Btrfs: fix listxattrs not listing all xattrs packed in the same item"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>