Remove TestGrantViaExitNodeSteering and TestGrantViaMixedSteering.
Exit node traffic forwarding through via grants cannot be validated
with curl/traceroute in Docker containers because Tailscale exit nodes
strip locally-connected subnets from their forwarding filter.
The correctness of via exit steering is validated by:
- Golden MapResponse comparison (TestViaGrantMapCompat with GRANT-V31
and GRANT-V36) comparing full netmap output against Tailscale SaaS
- Filter rule compatibility (TestGrantsCompat with GRANT-V14 through
GRANT-V36) comparing per-node PacketFilter rules against Tailscale SaaS
- TestGrantViaSubnetSteering (kept) validates via subnet steering with
actual curl/traceroute through Docker, which works for subnet routes
Updates #2180
b762e4c350