test: adjust luajit-tap testing machinery

This changeset makes possible to run luajit-tap tests requiring
libraries implemented in C:
* symlink to luajit test is created on configuration phase instead of
  build one.
* introduced a CMake function for building shared libraries required for
  luajit tests.

Furthermore this commit enables CMake build for the following luajit-tap
tests:
* gh-4427-ffi-sandwich
* lj-flush-on-trace

Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Reviewed-by: Sergey Ostanevich <sergos@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>