[test/querier] use the new connection api (part 1/3)
In devtools/repotest, we provide a simple `.qexecute` with autocommit,
and we use it (where possible) in unittest_querier.
Note that the .session object is a real session, not the magic property
found in CubicWebTC test classes.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')