test: add coverage for mknodat and ioprio_get/ioprio_set

Add end-to-end integration scenarios and tests for two previously
untested syscalls:

- mknodat(2): new dir-mknodat-fifo scenario creates an unprivileged
  FIFO node (S_IFIFO, no CAP_MKNOD) via unix.Mknodat under AT_FDCWD
  and unlinks it. TestDirMknodatFifo asserts enter_mknodat fires with
  pathname@args[1] (after dirfd@args[0]), proven by a PathContains
  match on the distinct fifo name, mirroring the mkdirat coverage.

- ioprio_get(2)/ioprio_set(2): new ioprio-basic scenario (the
  I/O-priority analogues of getpriority/setpriority) issues the raw
  syscalls (no x/sys wrapper exists), reading the current self I/O
  priority and re-applying it, or a harmless unprivileged best-effort
  value when none is set. TestIoprioBasic asserts enter_ioprio_get and
  enter_ioprio_set fire (null enters, UNCLASSIFIED ret), mirroring
  priority-basic. Realtime class is deliberately avoided as it needs
  CAP_SYS_ADMIN.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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