tests: Create internal library for unit tests

tests: Create internal library for unit tests

Our shared library only exports very few symbols so that we can change
our internal ABI easily and presumably not many applications are going
to link against libpakfire anyways.

This makes it impossible to test any internal functions which are
inaccessible. This patch adds an internal library with all available
symbols which will be linked into the test binaries so that we can test
those functions without any acrobatics.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>