Refactors test setup and teardown logic to use `t.Cleanup` instead
of `defer`. This ensures that cleanup functions are correctly scoped
to each subtest's lifecycle, improving test isolation and reliability.
The `setUpNetlinkTest` helper function is also improved to correctly
save and restore the original network namespace, ensuring that tests
do not leak state.
To support this, a `Close()` method that returns an error is added to
the `Handle` struct, allowing for proper cleanup of underlying netlink
sockets. The test helpers are updated to use this new method,
preventing resource leaks between tests.
Additionally, a bug in the `netns` tests is fixed where a large
namespace ID could overflow a 32-bit integer, causing spurious
failures on some systems.
The xfrm framework is linux-only. Only implement the respective types
for GOOS=linux to avoid dependencies to x/sys/unix on non-linux or
non-unix platforms. Provide dummy XfrmPolicy and XfrmState types for the
globally defined XfrmPolicy* and XfrmState* functions.
