[test] Skip LDAP tests if any of the underlying programs is not found
Some programs are in ldap-utils or ldapscripts, which may not be installed
despite slapd may.
Follow-up on 282880f81311.
usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/