Fix suspicious benchmark results

[kubouch]

Currently, on my PC, the combined1000_nu_old benchmarks takes about 3-4 ms, which gives the old parser is very fast. However, when parsing the same file in Nushell (using the nu-check command which pretty much just calls the parser), it takes about 180 ms. So, our benchmarks are probably not set up correctly.

[kubouch]

I tried using std::hint::black_box() to no avail. But I'm not very experienced with Rust benchmarking in general, I likely missed something.