Mocked polling loop implementation for testing

It would be nice if we could completely remove time as an input to our testing of cppTango. This will make the testing much more deterministic and reliable on CI.

One big sauce of timing problems when the tests are run on CI is the polling loop. If we make the polling loop configurable, we could provide a mock implementation for the cppTango tests which where "time is driven by the tests".

The mock implementation could add a dynamic DServer command to trigger the polling of a particular work item. The mock polling loop itself doesn't do anything automatically. Then the test could set up the polling for the test scenario, trigger the poll itself manually, then test that the expected behaviour was observed.

In most tests where we involve the polling loop, we just wait until we get the event and don't care about the internals of the polling loop scheduling.