cubicweb-dataio #5654901 skip tests requiring rdflib/librdf if they're not installed [validation pending]

skip tests requiring rdflib/librdf if they're not installed

prioritynormal
typebug
done in0.6.1
load left0.000
closed by#266a624aae50 skip tests requiring rdflib/librdf if they're not installed
patchskip tests requiring rdflib/librdf if they're not installed [applied]