tests: net: mld: Update after solicited-node mcast refactor.

1. MLD events now can happen before the test starts to run, (at iface
initialization time), so use static initialization of test semaphore.
2. Don't use a well-known multicast group like ff02::1 (all nodes),
as they are managed by the underlying stack. Use a dedicated test
address (ff10::1 here).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>