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.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2020 Purism SPC
+ * SPDX-License-Identifier: LGPL-2.1+
+ * Author: Guido Günther <agx@sigxcpu.org>
+ */
+#include "libfeedback.h"
+
+static gboolean          _initted;
+
+
+gboolean
+lfb_init (const gchar *app_id, GError **error)
+{
+  _initted = TRUE;
+  g_debug ("libfeedback mock library initialized");
+  return TRUE;
+}
+
+void
+lfb_uninit (void)
+{
+  _initted = FALSE;
+}
+
+gboolean
+lfb_is_initted (void)
+{
+  return _initted;
+}