[devtools] make CubiwebTC.login() context-manager restore the testcase connection on exit

[devtools] restore ability to test on real databases

[migration] fix add_relation_type() for symmetric relations To check if the corresponding SQL table has to created, CWRelationAddOp tests that the tablename is not in the 'createdtables' session variable (i.e. the table hasn't been created before in the same transaction) and that len(rschema.rdefs) is not greater to 1, that is the relation already existed in the database before this transaction. In case of symmetric relations, len(rschema).rdefs will be at least 2 since rdefs will store both (subj, obj) and (obj, subj) relation definitions.