test: adjust the path to LuaJIT test directory

To ease the further maintenance of the tests located in LuaJIT repo,
particular hardcoded paths are replaced with the root path to test
directory within LuaJIT third party submodule.

Closes #5425

Reviewed-by: Timur Safin <tsafin@tarantool.org>
Reviewed-by: Alexander V. Tikhonov <avtikhon@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>