Cleanup usage of StdRng::new and task_rng

I replaced StdRng::new with task_rng in a few tests and benchmarks (not
in the parts being benchmarked only in the setup). This will make
benchmarks and tests go by slightly faster.

I also added a missing benchmark for task_rng.

I didn't replace the StdRng::new with task_rng in test/bench/noise.rs
line 44 because the StdRng::new part is being measured by the
benchmark. I think the StdRng::new() part is a nonessential part of the
benchmark though so maybe it should be replaced.
auto merge of #13111 : sstewartgallus/rust/cleanup-stdrng-new, r=huonw

I replaced StdRng::new with task_rng in a few tests and benchmarks (not
in the parts being benchmarked only in the setup). This will make
benchmarks and tests go by slightly faster.

I didn't replace the StdRng::new with task_rng in test/bench/noise.rs
line 44 because the StdRng::new part is being measured by the
benchmark. I think the StdRng::new() part is a nonessential part of the
benchmark though so maybe it should be replaced.