[netstack] add http performance test

[dpeckett]

With a 1500 byte MTU

=== RUN   TestTunDevice_Speed
2025/08/15 08:38:21 INFO Adding protocol address addr=10.1.0.1/24
2025/08/15 08:38:21 INFO Adding addr to SNAT addr=10.1.0.1/24
2025/08/15 08:38:21 INFO Adding protocol address addr=10.1.0.2/24
2025/08/15 08:38:21 INFO Adding addr to SNAT addr=10.1.0.2/24
=== RUN   TestTunDevice_Speed/Speed
2025/08/15 08:38:21 DEBUG SNAT target selected address address=10.1.0.2
    tun_device_test.go:187: 1 stream: 209715200 bytes in 2.849174258s → 73.61 MB/s, 588.84 Mbit/s (0.59 Gbit/s)
=== RUN   TestTunDevice_Speed/SpeedParallel
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:24 DEBUG SNAT target selected address address=10.1.0.2
    tun_device_test.go:242: 8 streams × 209715200 bytes: 1677721600 bytes in 14.753897515s → 113.71 MB/s, 909.71 Mbit/s (0.91 Gbit/s)
--- PASS: TestTunDevice_Speed (17.61s)
    --- PASS: TestTunDevice_Speed/Speed (2.85s)
    --- PASS: TestTunDevice_Speed/SpeedParallel (14.75s)
PASS
ok      github.com/apoxy-dev/apoxy/pkg/netstack 17.612s

with a 9000 byte MTU (jumbo frames)

=== RUN   TestTunDevice_Speed
2025/08/15 08:38:05 INFO Adding protocol address addr=10.1.0.1/24
2025/08/15 08:38:05 INFO Adding addr to SNAT addr=10.1.0.1/24
2025/08/15 08:38:05 INFO Adding protocol address addr=10.1.0.2/24
2025/08/15 08:38:05 INFO Adding addr to SNAT addr=10.1.0.2/24
=== RUN   TestTunDevice_Speed/Speed
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
    tun_device_test.go:187: 1 stream: 209715200 bytes in 614.408944ms → 341.33 MB/s, 2730.63 Mbit/s (2.73 Gbit/s)
=== RUN   TestTunDevice_Speed/SpeedParallel
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
2025/08/15 08:38:05 DEBUG SNAT target selected address address=10.1.0.2
    tun_device_test.go:242: 8 streams × 209715200 bytes: 1677721600 bytes in 2.861272939s → 586.35 MB/s, 4690.84 Mbit/s (4.69 Gbit/s)
--- PASS: TestTunDevice_Speed (3.48s)
    --- PASS: TestTunDevice_Speed/Speed (0.61s)
    --- PASS: TestTunDevice_Speed/SpeedParallel (2.86s)
PASS
ok      github.com/apoxy-dev/apoxy/pkg/netstack 3.485s