Investigate mocking PyTango devices for testing

Based on discussion between Team Karoo with Giorgio and Vivek, Giorgio's resulting agreed-on suggestion:  provide well written examples of test cases in the context of the CentralNode case study that exemplify the problem of creating mocks/patches of tango devices so that we have device A that interacts with device B, and we create a mock for device B where "interacts with" means that A sends a command to B, and A reads an attribute of B, and A subscribes to change events in B and so that the examples cover all these possibilities. with code that is based on synchronous communication; let's leave asynch code for later with at least 1 example that uses also  s.williams 's technique based on monkeypatch ( https://docs.pytest.org/en/latest/monkeypatch.html) With the TCoP coordinator write a confluence page that explains these examples. It would be good to include in these pages some of the material that was already produced wrt to DeviceTestContext. Initial acceptance criteria: there are examples of tango devices whose proxies are mocked with pytest mock/patch, and the mocks are used to support unit testing of methods implementing commands, attributes, states of the device. some mocks are designed so to support testing of methods that are called according to an event-driven approach (eg on attribute changes). no interaction between tango devices is handled nor assumed by these tests. no interaction with Tango database is handled either.