Also add additional tests to verify that the unchecked implementation
is allocation-free.
Updates #12614
Signed-off-by: Nick Khyl <nickk@tailscale.com>
So that it works with both ctxlock.Context and context.Context without allocating
and without requiring ctxlock.Wrap at callsites.
Updates #12614
Signed-off-by: Nick Khyl <nickk@tailscale.com>