Skip SPARQLStore test if SPARQLStore is not installed

Trying to package RDFLib 4.1.0 on Fedora, I ran into a blocker with this test.
SPARQLStore is not yet packaged for Fedora, and unlike other tests which are
skipped if SPARQLStore is not installed, this one throws a fatal error. So
let's skip it like the others.

Signed-off-by: Dan Scott <dan@coffeecode.net>