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

- updated the tests for the testable mode and test the TestFacility class there as well