`x test tidy` can be faster

[onur-ozkan]

Right now, x test tidy includes two additional checks besides tidy itself: auto-completions and rustfmt check. Currently, they run sequentially, but running them concurrently could speed things up; potentially saving around one second depending on the machine specs. It's a quite popular and frequently used command, so it's worth making it faster.

Somewhat related: #141073

[chenyukang]

absolutely worth to improve here, on my machine it cost 16 seconds now:

➜  rust git:(up-master) x test tidy       
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.25s
Building stage0 tool tidy (aarch64-apple-darwin)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.39s
fmt check
fmt info: No modified files detected for formatting.
tidy check
x.py completions check
Build completed successfully in 0:00:16

[the8472]

One problem is that x is supposed to obey parallelism limits (-j) and both tidy and rustfmt individually respect that but via explicitly passed limits, not via jobserver support. So executing them in parallel would mean they'd not cooperatively limit themselves.