sai_test: add opt-in SIMULATE_SONIC to emulate SONiC teamd/IntfMgr netdevs for standalone benches

[nicholasching]

Context / motivation
Part of the SAIVPP unit-test framework (see the sonic-sairedis docker-sai-test-vpp harness). When the OCP sai_test T0 suite runs against a SAI backend on a standalone bench with no SONiC control plane, there is no teamd to create PortChannel netdevs and no IntfMgr to assign LAG/SVI interface IPs — which some backends (e.g. the VPP virtual switch) rely on for LAG bring-up and routed-to-LAG/SVI forwarding. This adds an opt-in way for the test setup to emulate that.

What this change does

• test/sai_test/config/simulate_sonic.py (new). Opt-in helper, active only when SIMULATE_SONIC=1; every entry point is a no-op otherwise.
• test/sai_test/config/lag_configer.py — create_lag(). When enabled, create the PortChannel<N> netdev teamd would provide, before the SAI LAG create.
• test/sai_test/config/route_configer.py — create_router_interface(). When enabled, assign the connected IPs IntfMgr would set on a LAG or VLAN(SVI) router interface.
• Vendor-neutral by construction. PortChannel netdevs use generic Linux ip … type bond; LAG RIF IPs use ip addr add. A VLAN SVI has no host-interface netdev to mirror from, so its address is programmed via command templates supplied by the caller (SVI_RIF_SET_IP_CMD / SVI_RIF_PROBE_CMD, with {ifname}/{addr} placeholders) — no backend-specific tooling is hardcoded in sai_test. All interface-name prefixes and address patterns are env-overridable.

Scope / risk

• 3 files (one new module + two setUp-path hooks) — test-config helpers only; no change to SAI/backend code or packet semantics.
• Fully opt-in with default-off: with SIMULATE_SONIC unset, all entry points no-op, so real-HW and other OCP consumers are unchanged unless they opt in.

Dependencies
None. Independent of #2299 / #2300 / #2301 (edits different functions); targets master.

[nicholasching]

@kcudnik, could you please invoke azure pipelines to run checks, please. I do not have the permissions to do so.

[kcudnik]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).