test: tarantoolctl: verify delayed box.cfg()

app-tap.tarantoolctl.test.lua fails after
17df9edf ('tarantoolctl: allow to start
instances with delayed box.cfg{}').

The commit fixes the test case that did check that an error is reported
if box.cfg() was not called in an instance script.

Follows up #4435.
Fixes #4448.

(cherry picked from commit 6c627af3)