L2SS-939: Use native mock library instead of external dependency

tangostationcontrol/requirements.txt

tangostationcontrol/test/beam/test_delays.py

tangostationcontrol/test/clients/test_attr_wrapper.py

tangostationcontrol/test/devices/device_base.py

...ostationcontrol/test/devices/interfaces/test_hierarchy.py

...stationcontrol/test/devices/sdp/test_statistics_device.py