tests: Add ringer tests

We mock libfeedback and CallsCall to test the ringer for the following
scenarios:
- Call state changes
- User requested silencing of ringer
- Multiple calls

tests/mock-call.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2021 Purism SPC
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Author: Evangelos Ribeiro Tzaras <devrtz@fortysixandtwo.eu>
+ *
+ */
+
+#pragma once
+
+#include "calls-call.h"
+
+G_BEGIN_DECLS
+
+#define CALLS_TYPE_MOCK_CALL (calls_mock_call_get_type())
+
+G_DECLARE_FINAL_TYPE (CallsMockCall, calls_mock_call, CALLS, MOCK_CALL, CallsCall)
+
+CallsMockCall    *calls_mock_call_new               (void);
+void              calls_mock_call_set_id            (CallsMockCall *self,
+                                                     const char    *id);
+void              calls_mock_call_set_name          (CallsMockCall *self,
+                                                     const char    *name);
+void              calls_mock_call_set_state         (CallsMockCall *self,
+                                                     CallsCallState state);
+void              calls_mock_call_set_inbound       (CallsMockCall *self,
+                                                     gboolean       inbound);
+
+G_END_DECLS