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/gpio_handle.cpp b/gpio_handle.cpp
index 2a46ae2..d99a031 100644
--- a/gpio_handle.cpp
+++ b/gpio_handle.cpp
@@ -12,8 +12,8 @@
 
 using namespace phosphor::logging;
 
-std::unique_ptr<gpioplus::Handle> BuildGpioHandle(const std::string& gpiochip,
-                                                  const std::string& line)
+std::unique_ptr<gpioplus::HandleInterface>
+    BuildGpioHandle(const std::string& gpiochip, const std::string& line)
 {
     char *gpioEnd, *lineEnd;
     unsigned long chipId = std::strtoul(gpiochip.c_str(), &gpioEnd, 10);