Ban direct calling of box.cfg()

[Totktonada]

It likely badly influence other tests and may look like luatest itself is buggy.

The proposal is to forbid calling box.cfg() not only from topmost level code, but also from tests. If we'll do only the former, a test that follows one that calls box.cfg() will be influenced and this effect will depend on the order of tests. It is better if a test with box.cfg() will always fail.

It can be implemented either as a check after require('<...>_test.lua') and after execution of a test. Or we can simply do box.cfg = nil before loading tests.

If box.cfg() is required for testing, it should be called on an auxiliary instance.

Motivating issue in tarantool/tarantool#7647. See also #191.

[Totktonada]

Just box.cfg = nil doesn't work (because the indexing is handled by box's metatable). However, we still can forbid it:

tarantool> box.cfg = setmetatable({}, {__call = function() error('No!', 0) end})
tarantool> box.cfg()
---
- error: No!
...

@ylobankov proposed to just add it to test.helper (a module that is required by luatest automatically) into tarantool repository. If it works, this is a good variant: easy to copy-paste by ones who need it, don't hurt simple test suites, where it may be convenient to call box.cfg() directly.

[ylobankov]

Let's consider this fixed. We banned calling box.cfg() in tarantool tests via tarantool/tarantool#8709. Anyone can do the same for her project as well.