nproc: nproc-quota.sh?: Rejects schedular

[oech3]

#9893 (comment)

[ChrisDryden]

@sylvestre unrelated to this specific issue but how would you feel about allowing commits that implement the rust integration tests for these scenarios but skip then with a link to this issue to help accelerate the process in fixing these.

I was thinking it would help out in reviewing since it would make the pr's smaller and help new contributors be able to have a better goal when making fixes

[oech3]

At least, #7241 (comment) should be replaced by helper under unshare -rm.

[oech3]

Better command (no syscall hijack)

$ run0 systemd-run --scope -q -p CPUQuota=100% chrt --fifo 10 gnu-nproc
4
$ run0 systemd-run --scope -q -p CPUQuota=100% chrt --fifo 10 uu-nproc
1