- use DummyRegister::getRawAccessor for access to backend registers.
- read and write convenience methods for rawAccessor.

Introduced UpdateModule which provides a Process Variable connected to
ExceptionDummy register  "REG1"

fixture_with_poll_and_push_input exposes pollVariable, pushVariable as
refernce types for convenience.

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.