test(migrate_pages): lock in UNCLASSIFIED return classification

Audit of migrate_pages(2) confirmed its tracing classification matches
the man page and its NUMA siblings:
- KindNull (null_event): args are pid (a pid, NOT an fd), maxnode, and
  two userspace bitmask pointers; the BPF handler emits a null_event and
  never reads args[0] as an fd.
- FamilyMemory: consistent with set_mempolicy/mbind/move_pages/
  set_mempolicy_home_node (the lone get_mempolicy->Security inconsistency
  is tracked separately and out of scope here).
- exit UNCLASSIFIED (ret_event): the return is the count of pages that
  could NOT be moved (>=0) or -1, a page tally rather than a transferred
  byte count.

Add explicit lock-in assertions for migrate_pages and its sibling
move_pages to TestClassifyRetUnclassified so a future stray
read/write/transfer reclassification trips the test.

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

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