testEphemeralData

Tests that, within a single query set_ephemeral(key, 1), (get_ephemeral(key, X) ; get_ephemeral(key, X)), the first get_ephemeral/2 succeeds binding X to 1, but backtracking into the disjunction to retry get_ephemeral/2 a second time fails: ephemeral data does not survive backtracking within the same query.