selftests: test_vxlan_fdb_changelink: Convert to lib.sh

Instead of inlining equivalents, use lib.sh-provided primitives.
Use defer to manage vx lifetime.

This will make it easier to extend the test in the next patch.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>