CMakeLists refactoring related to unittests

• Use a common function for defining unit tests.
• Remove all download scripts etc around foursources.reg: Simply put it inside the repository.
• Let each module install its own public headers.

cmake/config.h.in

-// Copyright (C) 2020 ASTRON (Netherlands Institute for Radio Astronomy)
-// SPDX-License-Identifier: GPL-3.0-or-later
-
-#ifndef SCHAAPCOMMON_CONFIG_H_
-#define SCHAAPCOMMON_CONFIG_H_
-
-#define DS9_REGIONS_FILE_MOCK "@DS9_REGIONS_FILE_MOCK@"
-
-#endif