fixture_with_poll_and_push_input exposes pollVariable, pushVariable as
refernce types for convenience.

TriggerFanOut is waiting for the device to open before starting the main loop to avoid logic_error in TransferGroup::read()

PollModule and PushModule now uses a void type promise to signal
mainLoop entry. The fixture class uses this to synchronize itself before
handling over control to the testcase code.

This way we do not have to depend on the test facility for this purpose.

Fixture with poll and push inputs can be reused in testing Exception handling.

Only the blocking read in wfnd TEs will update the module's version