The libdnf5 library unit tests use the private methods of the tested
libdnf5 library. The `BaseTestCase` class used as a parent of many test
scenarios, not only libdnf5 but also libdnf5-cli and other unit tests,
provides the `add_system_pkg` method, which also uses private methods
of the libdnf5 library. Because the private methods have hidden symbols,
they are not exported by the libdnf5 shared library. To access
the symbols of the private methods, unit tests link libdnf5 statically
instead of using the libdnf5 shared library. This is fine for unit tests
of the libdnf5 library. The problem is with unit tests of other
components (e.g. libdnf-cli) that link the shared libdnf5 library.
Due to this, in these tests the libdnf5 library was linked statically
because of used class `BaseTestCase` and at the same time the shared
libdnf5 library was linked.

The problem was solved by moving the `add_system_pkg` method from
the `BaseTestCase` class to the newly created inherited
`LibdnfPrivateTestCase` class, which is only used in the libdnf5 library
unit tests. Thus, only in these tests must libdnf5 be linked statically.
The tests of the other components still use the `BaseTestClass` class,
which now uses only public (API) functions from the libdnf5 library.
These tests now only link the shared version of the libdnf5 library.