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-contacts-provider.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include "calls-contacts-provider.h"
+
+CallsContactsProvider *__wrap_calls_contacts_provider_new (CallsSettings *seetings);