if the path environment variable is missing, add the tests for
  <shareable/> from/to w! ode for disk
* tests/sexpr2xmldata/sexpr2xml-disk-block-shareable.sexpr
  tests/sexpr2xmldata/sexpr2xml-disk-block-shareable.xml
  tests/xml2sexprdata/xml2sexpr-disk-block-shareable.sexpr
  tests/xml2sexprdata/xml2sexpr-disk-block-shareable.xml:
  the new tests data
Daniel