Add the script body parameter into test_run:create_server()

[Totktonada]

Example:

test_run:create_server(<name>, {
    <..other options are here..>
    script = [[
        box.cfg{
            listen = os.getenv('LISTEN'),
        }
        require('console').listen(os.getenv('ADMIN'))
    ]],
})
test_run:start_server(<name>, {<startup options>})

It is easier to have everything within a test file rather than spread across different files.

NB: There is no test_run:create_server(<...>) function yet, only test_run:cmd('create server <...>'). We should add the wrapper function to properly escape the script body. And, so, start_server(), restart_server(), stop_server(), cleanup_server(), delete_server() as well.

The idea was described first in #173 (comment).

---

There are thoughts around a way we want to use to start instances. Now we use tarantoolctl, iproto and console ports assigned by test-run. However, maybe, it would be more obvious to run 'pure tarantool' scripts. I don't know, to be honest.