Improve MT RADIUS unit tests

Improve RADIUS unit tests:

• no goal: write tests for session history: instead implement #414
• MT: see below
• async access added new ways to have a query to be dropped: add these cases
• find a way to detect accounting exchange termination (e.g. a class counter of pending exchanges)

RADIUS could have more thorough MT unit tests:

• Start thread pool for accounting by calling the dhcp*_srv_configured callout. Currently it is only called for auth. Waiting for work to finish in accounting is not as trivial for auth. Auth uses the unparking for that. (see last general point)
• In both access and accounting, start a second thread pool that simulates the core Kea thread pool / DHCP clients.
• Convert more (all?) ST unit tests to MT. Currently there are only 4 MT unit tests: (v4 + v6) x (access + accounting).