system: Installation progress notifications

Implement installation progress notifications as defined in
czechlight-system model
(/czechlight-system:firmware/installation/update).

The RAUC D-Bus API invokes PropertyChanged signal on its Progress [1]
property whenever the installation progresses. We capture those signals
and use them to generate Sysrepo notifications to clients.

The update contains current progress (in percents) and a message
containing current installation stage.

[1] https://rauc.readthedocs.io/en/v1.4/reference.html#gdbus-property-de-pengutronix-rauc-installer-progress

Change-Id: I310752919dde523a93943a2a1d7af79f073aedfe
diff --git a/tests/test_sysrepo_helpers.h b/tests/test_sysrepo_helpers.h
index bf8bf5f..54d2d71 100644
--- a/tests/test_sysrepo_helpers.h
+++ b/tests/test_sysrepo_helpers.h
@@ -44,6 +44,7 @@
     auto srSess = std::make_shared<sysrepo::Session>(srConn); \
     auto srSubs = std::make_shared<sysrepo::Subscribe>(srSess);
 
-#define TEST_SYSREPO_INIT_CLIENT                               \
-    auto clientConn = std::make_shared<sysrepo::Connection>(); \
-    auto client = std::make_shared<sysrepo::Session>(clientConn);
+#define TEST_SYSREPO_INIT_CLIENT                                  \
+    auto clientConn = std::make_shared<sysrepo::Connection>();    \
+    auto client = std::make_shared<sysrepo::Session>(clientConn); \
+    auto subscription = std::make_shared<sysrepo::Subscribe>(client);