Move plugin specific tests into dedicated directory

This will prove beneficial when we also add tests for the policy engine
plugins. The increased locality is also nice to have.

plugins/provider/tests/setup-provider.h

@@ -0,0 +1,14 @@
+/*
+ * Copyright (C) 2018 Purism SPC
+ *
+ * SPDX-License-Identifier: GPL-3.0+
+ */
+
+#include "calls-dummy-provider.h"
+
+typedef struct {
+  CallsDummyProvider *dummy_provider;
+} ProviderFixture;
+
+void test_dummy_provider_set_up (ProviderFixture *fixture, gconstpointer user_data);
+void test_dummy_provider_tear_down (ProviderFixture *fixture, gconstpointer user_data);