bench should put deliminators on nanosecond numbers

[mdinger]

From this initial rust experience blog, cargo bench (I assume identical to rustc --test) should report nanosecond times as 3_000_000 or 3,000,000, not 3000000.

This reportedly would require fixing in libtest.

[bluss]

I posted a PR for this yesterday. Well I chose to round & use units, but you can opine either way #26068

[mdinger]

@bluss nice! If you'll just tag that as fixing this then bors will take care of the rest.