Add an automated test for dal.isMember()

See https://forum.duniter.org/t/bug-a-l-ajout-d-une-certification-d-un-non-membre-dans-le-bloc-a-calculer-v1-7-16/5952

isMember() should work: return true when and identity is member, false when it’s not

Tests to lock this feature fixed in f9778328