Add a deterministic regression test for the Android/OpenHarmony ethtool speed truncation #136353

SystemNative_GetNetworkInterfaces decoded only the low 16 bits of ethtool_cmd.speed on Android, so speeds above 65535 Mbps were truncated (100 Gbps -> 34.464 Gbps). The OpenHarmony path had the same decoding issue; both are fixed in #136194 (follow-up to #135866).

The bug only manifests with a NIC reporting >65535 Mbps, and the managed tests cannot inject an ethtool response. A local harness that compiles the real pal_interfaceaddresses.c with -DTARGET_ANDROID and mocks getifaddrs()/ioctl() reproduces it deterministically:

• before: 34,464,000,000 bit/s
• after: 100,000,000,000 bit/s

Harness and output: https://gist.github.com/springmin/772d3adc5ccea004c7116a59c04bedb9

Proposal:

1. Add the harness as a native unit test under src/native/libs/System.Native/tests (the directory does not exist today) and wire it into the build/test infrastructure.
2. Alternatively, add a conditional /sys/class/net/<iface>/speed check in NetworkInterfaceBasicTest for Linux/Android.

This is a test-only follow-up and does not block #136194.