test: fix flaky config-luatest/validate_names_test

Sometimes the test fails with "Peer closed" error, logs says, that
fatal error happened: cfg_get('read_only'). This is caused by the
instance processing its own ballot.

The problem is that we set box.cfg to function in before_all trigger,
so it's impossible to access box.cfg table during the whole time of test
execution.

Let's instead set box.cfg to function at the start of every test and
restore box.cfg at the end. This way we'll decrease the time, in which
such fatal error can happen. Even though it's still possible to get it
in theory, the problem is not reproduced anymore. The alternative
solution of introducing errinj seems to be overhead here.

Closes tarantool/tarantool-qa#329

NO_DOC=testfix
NO_CHANGELOG=testfix