feat: add unit test suite

[podrivo]

Refs #591

Problem

The project has no automated test suite. The shared byte parser (lib/reader.js) and other core utilities are relied on by nearly all ~80 protocols, yet there's no safety net to catch regressions, no npm test script, and no CI job to run tests. See #591.

Fix

Introduce a lightweight, zero-dependency unit test suite covering core parsing and utility logic, an npm test script, and a CI job to run it. No production code is changed — this PR only adds tests and wiring.

Approach:

• Runner: Node's built-in node:test + node:assert/strict. No new dependencies, no build step, works with the existing ESM setup.
• Scope: fast, offline unit tests only — no live servers or network.
• Layout mirrors the source tree so tests are easy to find and scale:

tests/
  lib/
    reader.test.js
    hexutil.test.js
    results.test.js
    game-resolver.test.js
    promises.test.js
  protocols/
    core-utils.test.js   # protocols/core.js helpers
    quake3.test.js

• node --test auto-discovers **/*.test.js recursively, so the script and CI need no per-file wiring.

What's covered:

• lib/reader.js — the shared byte parser used by nearly every protocol: fixed-length & delimited strings, custom encodings (incl. the latin1→win1252 alias), pascalString, signed/unsigned ints (LE/BE, incl. 64-bit Long), floats, varints, part, and offset navigation (skip/setOffset/rest/done).
• lib/HexUtil.js — debugDump length reporting, hex rendering, control-byte masking.
• lib/Results.js — Player/Players/Results construction and defaults.
• lib/game-resolver.js — lookup(): protocol-* types, known ids, unknown ids, and checkOldIDs deprecated-id resolution.
• lib/Promises.js — createTimeout fires after the timeout and is silenced by cancel() (the primitive behind every socket/attempt timeout — see bug: Timeouts not triggering for TCP queries #611).
• protocols/core.js — pure helpers trueTest, translate, assertValidPort.
• protocols/quake3.js — stripColors (single-char ^1 and extended ^XRRGGBB color codes, plus the disabled passthrough).

CI: adds a test job to .github/workflows/node.yml running npm test on Node 18.x / 20.x / 22.x. (The built-in runner's auto-discovery is reliable on 18+; Node 16 stays covered by the existing build job.)

Examples

npm test            # node --test

Compatibility

• No production code is changed — this PR only adds tests and CI wiring, so it's fully backwards-compatible.
• Zero new dependencies and no build step (uses the built-in node:test runner with the existing ESM setup).
• This establishes the harness + a safety net for the shared parsing primitive (which all ~80 protocols rely on) + one example protocol test. It is the first batch, not full coverage.

Follow-ups (out of scope for this PR):

• Per-protocol parsing tests: extract each protocol's parse step into a pure parseX(buffer, state) method, then add captured-buffer fixtures under tests/protocols/ (open bug reports are good fixture sources).
• CLI host/port parsing incl. IPv6 (needs the parser extracted out of bin/gamedig.js).
• A separate integration suite for live-query behavior (timeouts, retries, multi-packet reassembly, e.g. bug: Timeouts not triggering for TCP queries #611 / bug: GameSpy 3 parsing breaks if server info is split across packets #708).

Testing

• npm test → 7 test files, 16 suites, 55 tests, all passing.
• npm run lint:check currently reports pre-existing failures on master in untouched files (the .eslintrc.json ecmaVersion: 2021 predates ES2022 syntax already in the code). This PR introduces zero new lint errors; fixing the pre-existing ones is a separate change.