hwpmc: add tests for handle validation and the privilege boundaries

A pmc_id_t is a packed integer that the driver hands to userland and
accepts back on eleven operations, and nothing tested what ha

hwpmc: add tests for handle validation and the privilege boundaries

A pmc_id_t is a packed integer that the driver hands to userland and
accepts back on eleven operations, and nothing tested what happens when
one comes back forged, stale, or belonging to another process. Neither
was there a test that an unprivileged caller is refused the operations
that need a privilege.

The cases use a SOFT-class PMC wherever the counter itself does not
matter, so they run on a machine with no PMU.

MFC after: 1 month
MFC to: stable/15
MFC to: stable/14
Assisted-by: Claude Code (Opus 5)

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