[libc] SCHED_FIFO and SCHED_RR tests fail on Arch Linux

[overmighty]

These tests fail on Arch Linux, where setting these scheduling policies succeeds even when not running as root.

See #95562.

[overmighty]

It turns out that this is not inherently related to Arch Linux, but to usage of the realtime-privileges package and running the test as a user part of the realtime group, which allows the user to set real-time scheduling policies.