Migrate `justrun` module and adapt it

[ochaplashkin]

The original justrun module (path: tarantool/test/justrun.lua) has been moved to the current project with minor changes and will be available as follows:

    local t = require('luatest')
    t.justrun.tarantool(...)

This module works with the popen module which requires Tarantool 2.4.1 and newer. Otherwise justrun.tarantool(dir, env, args[, opts]) will cause an error.

Closes #365