Implemented sensor class

Sensor class was introduced, it monitors
xyz.openbmc_project.Sensor.Value, for change and notifies all
listeners.

Tested:
  - Unit tested with service stub that provides dbus interface
    xyz.openbmc_project.Sensor.Value
  - All changes are delivered to listeners
  - All other unit tests are passing

Change-Id: I8c9d58cc986c1fe2a4d2386815d559814016efa6
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/tests/src/mocks/sensor_mock.hpp b/tests/src/mocks/sensor_mock.hpp
index 9e8d7f3..0dcb2b5 100644
--- a/tests/src/mocks/sensor_mock.hpp
+++ b/tests/src/mocks/sensor_mock.hpp
@@ -20,7 +20,9 @@
         return Id("SensorMock", service, path);
     }
 
-    MOCK_CONST_METHOD0(id, Id());
+    MOCK_METHOD(Id, id, (), (const, override));
+    MOCK_METHOD(void, registerForUpdates,
+                (const std::weak_ptr<interfaces::SensorListener>&), (override));
 
     const uint64_t mockId = generateUniqueMockId();