test: adapt tests to the new luatest version

With the new version of Luatest you have to be careful with the server
log file. We used to get it very simply:

    box.cfg.log

Now it is more correct to use the following approach:

    rawget(_G, 'box_cfg_log_file') or box.cfg.log

Closes tarantool/test-run#439

NO_DOC=test
NO_TEST=test
NO_CHANGELOG=test