tests: remove cflag -w and fix warnings

The flag hides errors. But we want unit tests to be correct in order
to reveal errors in the main code.

The patch removes tests_unittests_unittests_CFLAGS which makes autotools
use AM_CFLAGS instead. Therefore, unit tests are built with flags
derived from configure.ac.

tests/unittests/test_server_events.c

@@ -13,6 +13,8 @@
 #include "ui/ui.h"
 #include "ui/stub_ui.h"
 #include "xmpp/muc.h"
+#include "plugins/plugins.h"
+#include "ui/window_list.h"
 
 void console_shows_online_presence_when_set_online(void **state)
 {