test: add a helper for testing interactive mode

A basic (and pretty useless) example:

```lua
local it = require('test.luatest_helpers.interactive_tarantool')

local child = it.new()

child:execute_command('6 * 7')
local res = child:read_response()
t.assert_equals(res, 42)

child:close()
```

The module also contains `:read_line()`, `:assert_line()` helpers for
testing output directly: for example, when we need to catch a print()
from a background fiber. It provides has constants related to terminal's
control sequences.

A real usage can be seen in a next commit.

Part of #7169

NO_DOC=no user visible changes
NO_TEST=not applicable, it is a testing helper
NO_CHANGELOG=no user visible changes