hwpmc: add regression tests for counting-PMC counter wraparound

[netchild]

Summary

Exercise a process-mode counting PMC whose accumulated count crosses,
or already exceeds, the range of the underlying hardware counter.
Before the previous commit, the first context switch after the
hardware counter wrapped panicked INVARIANTS kernels with "negative
increment" and silently corrupted the accumulated count on other
kernels.

The tests need a hardware counting event backed by a counter narrower
than 64 bits and skip where none is available (hwpmc(4) not loaded,
or a VM without a vPMU).

MFC after: 2 weeks
Assisted-by: Claude Code (Fable 5)