tests: Mock libfeedback

Copied from libfeedback as of 2e081602f627505e566cc0bdb6cd96c7642d8b65
and adjusted for our mocking needs.

The mocked library will be LD_PRELOADED for the moment,
but further changes to the build should allow us to simply link
to it in the future.

tests/mock/lfb/libfeedback.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (C) 2020 Purism SPC
+ *
+ * SPDX-License-Identifier: LGPL-2.1+
+ */
+#pragma once
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+#include "lfb-enums.h"
+#include "lfb-event.h"
+
+gboolean    lfb_init (const gchar *app_id, GError **error);
+void        lfb_uninit (void);
+gboolean    lfb_is_initted (void);
+
+G_END_DECLS