...and package the ZombieCheck as helper object.
Also rewrite the SyncClassLock_test to perform an
multithreaded contended test to prove the lock is shared and effective
When some dependency or singleton violates Lumiera's policy regarding destructors and shutdown,
we are unable to detect this violation reliably and produce a Fatal Error message.
This is due to lib::Depend's de-initialisating being itself tied to template generated
static variables, which unfortunately have a visibility scope beyond the translation unit
responsible for construction and clean-up.
