Link nupic.core static lib tests with the nupic_core combined library instead of solo

[vitaly-krugl]

Presently, the static lib tests in nupic.core are linked with the nupic_core_solo static library, which is an intermediate library that is not intended to be an output artifact. The nupic_core "combined" static library is the intended static library output artifact. Therefore, the tests should be linked against and be testing the "combined" nupic_core library.