Fix tests libraries on macOS

[swaldhoer]

The test libraries can not be build, e.g., see Tracis CI:

• https://travis-ci.org/swaldhoer/native-lua/jobs/570248676#L528-L543

[37/77] Linking build/clang/tests/libs/lib2.dylib
Undefined symbols for architecture x86_64:
  "_luaL_checkversion_", referenced from:
      _luaopen_lib2 in lib2.c.13.o
  "_luaL_setfuncs", referenced from:
      _luaopen_lib2 in lib2.c.13.o
  "_lua_createtable", referenced from:
      _luaopen_lib2 in lib2.c.13.o
  "_lua_gettop", referenced from:
      _id in lib2.c.13.o
  "_lua_setglobal", referenced from:
      _luaopen_lib2 in lib2.c.13.o
  "_lua_settop", referenced from:
      _luaopen_lib2 in lib2.c.13.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

These libraries must be built in order to run the full tests