tests: compile some tools/tests as C++

These tests use things from the common libs, or at least include header
files from src/common.  These files are going to contain C++-specific
things in a following commit, so it's easier if we compile them
tools/tests as C++.

Change-Id: Ib99f2373beb414c50eaa10b35e0d895bc37e4e64
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>