test: add HandleInterface and mock

Add a handle mock for use by other daemon's unit-tests.  This saves a
separate daemon from using the SysMock.

Change-Id: I11802ca2f1b00e3e0de8966856c75599e560996e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/src/Makefile.am b/src/Makefile.am
index e366846..7bf0aa2 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -20,5 +20,6 @@
 libgpioplus_la_SOURCES += gpioplus/internal/sys.cpp
 
 nobase_include_HEADERS += gpioplus/test/sys.hpp
+nobase_include_HEADERS += gpioplus/test/handle.hpp
 
 nobase_include_HEADERS += gpioplus/utility/aspeed.hpp