hwpmc: add regression tests for a credential-changing exec

This tests what FreeBSD-SA-26:56.hwpmc fixed.

exec_setgid_drops_pmc asserts the kernel takes a process-mode PMC away
when its target execs

hwpmc: add regression tests for a credential-changing exec

This tests what FreeBSD-SA-26:56.hwpmc fixed.

exec_setgid_drops_pmc asserts the kernel takes a process-mode PMC away
when its target execs a set-gid program its owner is not entitled to
trace.

exec_setuid_no_double_unlink lets the target exec a set-uid program;
the teardown must unlink the process descriptor exactly once, and
completing at all is the assertion.

Both need an unprivileged owner and must not drop privileges themselves,
since p_candebug() would then refuse the target to its own owner; they
ask for require.user instead.

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

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