test: add gpiohandle mock impl

Add gpio handle mock implementation for use in testing.

Change-Id: I0fb2d1af4739ad700f62a4e7cbbe62db528bb93e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/sensor.hpp b/sensor.hpp
index 3bcfc82..bf98724 100644
--- a/sensor.hpp
+++ b/sensor.hpp
@@ -138,7 +138,7 @@
     valueAdjust sensorAdjusts;
 
     /** @brief Optional pointer to GPIO handle. */
-    std::unique_ptr<gpioplus::Handle> handle;
+    std::unique_ptr<gpioplus::HandleInterface> handle;
 
     /** @brief default pause after unlocking gpio. */
     static constexpr std::chrono::milliseconds pause{500};