tests: Fix test suite

Introduce a new GcalStubCalendar class that essentially
removes the ECalClient initialization from GcalCalendar,
and use it as part of a new private library consumed by
the tests.

tests/gcal-stub-calendar.h

@@ -0,0 +1,33 @@
+/* gcal-stub-calendar.h
+ *
+ * Copyright 2019 Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "gcal-calendar.h"
+
+G_BEGIN_DECLS
+
+#define GCAL_TYPE_STUB_CALENDAR (gcal_stub_calendar_get_type())
+G_DECLARE_FINAL_TYPE (GcalStubCalendar, gcal_stub_calendar, GCAL, STUB_CALENDAR, GcalCalendar)
+
+GcalCalendar*        gcal_stub_calendar_new                      (GCancellable       *cancellable,
+                                                                  GError            **error);
+
+G_END_DECLS