Tests: create new test cases to verify sensor APIs.

1. Create a dummy sensor driver and implement the sensor interfaces:
    .sample_fetch = &dummy_sensor_sample_fetch,
    .channel_get = &dummy_sensor_channel_get,
    .attr_set = dummy_sensor_attr_set,
    .trigger_set = dummy_sensor_trigger_set.

2. Create below two test cases to verify the sensor subsys APIs:
    test_sensor_get_channels() for verifying sensor channel APIs,
    test_sensor_handle_triggers for verifying sensor trigger APIs.

Signed-off-by: YouhuaX Zhu <youhuax.zhu@intel.com>