Fix a logic error in IpServerTest#addRemoveipv6ForwardingRules

When checking that link-local and multicast neighbours are
ignored, make sure the test neighbours are added on the correct
interface. Otherwise, they might be ignored because events on the
wrong interface are ignored, and not necessarily because
link-local and multicast neighbours are ignored.

Test: atest TetheringTests
Change-Id: I4a624ea4ce9ee9a9352afccbc7bf866587d4cdfa