cmake: don't include EXTRA_CORE_INCLUDE_DIRS in test/CMakeLists.txt

We need to include EXTRA_CORE_INCLUDE_DIRS in order to build unit tests.
Including it into test/CMakeLists.txt is useless. Moreover, it may break
EE build in case EXTRA_CORE_INCLUDE_DIRS includes a system directory
that contains a file that has the same name as one of our internal
headers.

See https://github.com/tarantool/tarantool-ee/issues/71

NO_DOC=cmake
NO_TEST=cmake
NO_CHANGELOG=cmake