locking: Method to check if LockSet is fully acquired

A new method is added to check whether the LockSet-internal lock is
held. This is the case after LockSet.acquire was called with
locking.ALL_SET.

Unit tests are updated, including one where the list of names must be
empty.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>