Split wxPersistenceManager-related tests and actually build them

It seems better to organize the tests in different files and just
provide a common fixture-like class to reuse functionality.

Also use this as an opportunity to rewrite the tests to use Catch
directly instead of using CppUnit-compatible macros.

Finally, actually build these tests as part of the test suite.

tests/persistence/testpersistence.h

@@ -0,0 +1,45 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        tests/persistence/testpersistence.h
+// Purpose:     Fixture for wxPersistentObject unit tests
+// Author:      wxWidgets Team
+// Created:     2017-08-23
+// Copyright:   (c) 2017 wxWidgets Team
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef WX_TESTS_PERSIST_TESTPERSISTENCE_H
+#define WX_TESTS_PERSIST_TESTPERSISTENCE_H
+
+#include "wx/app.h"
+#include "wx/config.h"
+
+#define PO_PREFIX           "/Persistent_Options"
+
+class PersistenceTests
+{
+public:
+    PersistenceTests()
+        : m_cleanup(false)
+    {
+        wxTheApp->SetAppName("wxPersistenceTests");
+    }
+
+    // The tests using this fixture should call this method when they don't
+    // need the values saved into wxConfig any more.
+    void EnableCleanup()
+    {
+        m_cleanup = true;
+    }
+
+    ~PersistenceTests()
+    {
+        if ( m_cleanup )
+        {
+            wxConfig::Get()->DeleteGroup(PO_PREFIX);
+        }
+    }
+
+private:
+    bool m_cleanup;
+};
+
+#endif // WX_TESTS_PERSIST_TESTPERSISTENCE_H