[dataimport] add tests for SQLGenObjectStore (closes #9177565)
Stolen from the massive import store, with the irrelevant bits removed.
Remove redundant tests from test_pgstore.py.
sync_schema_props_perms('cwuri')
helper = repo.system_source.dbhelper
cu = session.cnxset.cu
helper.set_null_allowed(cu, 'moved_entities', 'extid', 'VARCHAR(256)', False)
commit()