Rollup merge of #81833 - the8472:parallel-bootstrap-rustfmt, r=Mark-Simulacrum

Rollup merge of #81833 - the8472:parallel-bootstrap-rustfmt, r=Mark-Simulacrum

parallelize x.py test tidy

Running tidy on individual commits when rewriting git history was somewhat of an annoyance, so I have parallelized it a bit.

running `time ./x.py test tidy` with warm IO caches:

old:

```
real	0m11.123s
user	0m14.495s
sys	0m5.227s
```

new:

```
real	0m1.834s
user	0m13.545s
sys	0m3.094s
```

There's further room for improvement (<0.9s should be feasible) but that would require bigger changes.