benches: Added nitg to measured engines

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
benches: removed non generic-covariant languages from benches applying only on generic-covariant subtyping test

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
benches: changed middle index list calculation

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
benches: moved instantiation out of the main loop

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>