Autorequire luatest in all instances to simplify `:exec()` calls

[Gerold103]

Very often I see in the tests code like this:

g.replica:exec(function()
    require('luatest').assert(...)
end)

It is repeated for the same instances inside of a single test case over and over again sometimes. I want the instance default file to require luatest into a global variable. For example, luat. So as it could be used in all exec calls without annoying require('luatest') multiple times.

[Totktonada]

I would even call it just t. It is the quite usual pattern in tests and will make exec's code more consistent with a test code.

[H9660]

Its easy to solve. Assign it to me please.

[Totktonada]

@H9660 This task is not assigned to anyone at the moment. You can just propose an implementation in a pull request.