test(priority): add end-to-end coverage for getpriority/setpriority

getpriority/setpriority (FamilyProcess, KindNull enter, UNCLASSIFIED ret)
were untested end-to-end: no ioworkload scenario invoked them. Add a safe,
unprivileged, non-disruptive priority-basic scenario that reads the current
nice value via getpriority(PRIO_PROCESS, 0) and re-applies that exact value
via setpriority(PRIO_PROCESS, 0, currentNice) — a byte-for-byte no-op,
mirroring the existing schedRoundtripAffinity round-trip. Note getpriority
returns 20-nice, so the value is converted back before re-applying.

Register the scenario in scenarios.go and add TestPriorityBasic, which
asserts enter_getpriority and enter_setpriority appear as null_event enters
attributed to the ioworkload process. Enter-tracepoint presence is the right
check given KindNull/UNCLASSIFIED (no fd/path/bytes to assert). Coverage
only — classification verified correct in audits 6u and pz.

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

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