Add tests directory for soter_test target

[concatime]

Update CMakeLists to add tests directory with private headers for soter_test target.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed

[Lagovas]

wow, awesome) someone use cmake/CMakeLists.txt which we added more for internal usage and CLion integration (because main build system is make for us)
I tested locally and yes, without this patch soter tests fail with errors:

/themis/tests/soter/soter_asym_ka_test.c:17:10: fatal error: soter/soter_test.h: No such file or directory
   17 | #include "soter/soter_test.h"
      |          ^~~~~~~~~~~~~~~~~~~~

because we have hidden private headers. With the patch, it builds and runs successfully.

[ilammy]

Hey @concatime! Thanks for fixing up CMake! Indeeed, it's not used much for development or packaging. If I remember correctly, it was added some long time ago for better IDE support but it was not really maintained since then as no one really needed it.