selftests/rtnetlink.sh: add mngtempaddr test

Add a test to check the temporary address could be added/removed
correctly when mngtempaddr is set or removed/unmanaged.

Signed-off-by: Sam Edwards <cfsworks@gmail.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>