tests: net: ipv6: Add more test cases involving iface state changes

Add tests verifying valid IPv6 ND behaviors on interface state changes,
specifically:
* Verify RS is sent, and upon RA reception prefix and autoconf address
  added accordingly.
* DAD is triggered for autoconf, static and LL IPv6 addresses.
* IPv6 address is not used upon DAD conflict.
* IPv6 neighbor discovery works as expected.
* Multicast transmission/reception is functional after interface state
  changes.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>