test: ban direct calling of box.cfg()

Direct call and configuration of the runner instance is prohibited. Now
if you need to test something with specific configuration use a server
instance please (see luatest.Server module).

In-scope-of tarantool/luatest#245

NO_DOC=ban calling box.cfg
NO_TEST=ban calling box.cfg
NO_CHANGELOG=ban calling box.cfg

(cherry picked from commit fc3426d8)