docs(testing): add near-mock as in-process alternative to sandbox

[Kampouse]

What

Adds a short section to the Integration Testing guide presenting near-mock — an open-source (MIT) in-process NEAR runtime — as a complement to near-sandbox for cases where a full node is overkill.

Why

• Fast iteration & hermetic CI: executes unmodified contract wasm via wasmtime in milliseconds — no node, Docker, RPC, or fees. Deterministic replay by pinning seed / block timestamp / epoch / validators via env vars.
• Chain parity: implements the production host surface (~140 host fns: storage, registers, promise batch actions, validators, epochs) with protocol-accurate gas metering (V155 fee table, fuel-metered by wasmtime) and real crypto precompiles (ecrecover low-s, p256_verify, alt_bn128, keccak512, BLS12-381) using the same crates as nearcore — so contracts behave the same as on-chain.
• Mainnet forensics: near-mock snapshot <account> <state.bin> pulls a pinned-block state snapshot from any RPC and lets you call it offline — useful for debugging live contracts, diffing behavior across upgrades, and post-mortems.

Where

Inserted right after the sandbox intro note in smart-contracts/testing/integration-test.mdx, before Create Accounts — positioned explicitly as complementary (the Tip keeps near-sandbox as the recommendation for full-node-stack needs).

Happy to adjust placement, tone, or trim if maintainers prefer a different location (e.g. its own page under testing/ or a mention in the tools section).