The VoidAccessor comes along with the convenience classes VoidInput and VoidOutput,
and the TestFacility function getVoid()

to check whether stepApplication() can execute...

This was a left-over from times when no proper initial value propagation
and exception handling was implemented in ApplicationCore. Waiting until
the backends are open is anyway not sufficient, since the DeviceModule
needs to finish the recovery procedure. If any test would still rely on
this (anti-)feature, it would be subject to race conditions.

The wait is removed because it prevents important tests for the
exception handling specification. In particular, tests which
intentionally break devices during startup of the application would not
be implementable.

allows to control whether to wait for device recoveries in stepApplication

The default value was properly propagated but not visible to the test until a value was sent through the return channel.

- Exception handling decorator does not allow to leave testable mode before
  devices are up
- Test facility makes sure that testable mode counter is zero at the end of startApplication()

do not read initial values in TestFacility if testable mode is disabled, as this is subject to race conditions then

allow using the TestFacility without the testable mode, which can be useful for some tests of the framework itself

- treat the two directions of bidirectional variables properly in testable mode (i.e. as individual variables with each one counter)
- fix a data race in testable mode in case a bidirectional variable rejected a value (was not holding a required lock)

fixed testable mode for bidirectional variables: the return channel of control system to application variables must not increment the testableMode_counter

fixed bug that using poll-type transfers in testable mode could sometimes lead to stalled tests or assert failures.

Cache accessors obtained through TestFacility::getScalar() and TestFacility::getArray() by the application test code, to avoid the loss of the current value if a decorated accessor is obtained again.

detect stalled tests and print a list of variables which contain data not processed by the application