build: disable LUAJIT_ENABLE_PAIRSMM

Since this build flag has been removed as a result of reverting the
tarantool/luajit@d4e985a, its definition in the corresponding Tarantool
cmake file is irrelevant.

Furthermore, considering the breakage faced in #4770 the following tests
are introduced:
* the check whether space __pairs metamethod is set to space.pairs to
  create a Lua Fun iterator that handles __pairs manually underneath.
* the check whether pairs builtin behaviour doesn't change when __pairs
  is set e.g. on space object.

Follow-up #4560
Closes #4770

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