xfstests: add _require_attrs

xfstests: add _require_attrs

Add a new helper to check if extended attributes are supported.  It
errors out if any of the attr tools are not found, or if a filesystem
does not support setting attributes.

Remove the opencoded checks for the attr tools from various tests
now that we do them in common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>