add connection_get_jid()

Use a singleton `Jid` inside the connection instead of always re-creating
a `Jid` from the same string.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

tests/unittests/xmpp/stub_xmpp.c

@@ -58,6 +58,11 @@ void
 connection_disconnect(void)
 {
 }
+const Jid*
+connection_get_jid(void)
+{
+    return mock_ptr_type(Jid*);
+}
 const char*
 connection_get_fulljid(void)
 {