Add three tests that verify control plane behavior for grant policies:
- TestGrantViaSubnetFilterRules: verifies the router's PacketFilter
contains destination rules for via-steered subnets. Without per-node
filter compilation for via grants, these rules were missing and the
router would drop forwarded traffic.
- TestGrantViaExitNodeFilterRules: same verification for exit nodes
with via grants steering autogroup:internet traffic.
- TestGrantIPv6OnlyPrefixACL: verifies that address-based aliases
(Prefix, Host) resolve to exactly the literal prefix and do not
expand to include the matching node's other IP addresses. An
IPv6-only host definition produces only IPv6 filter rules.
Updates #2180
0431039f2a