Address review on the test harness

- The generic parts move to test/ in the repository root so other plugins can
  reuse them: the factorio and clusterio stubs, the test framework, the
  fengari runner, and clusterio's testMatrix and round trip helpers. A plugin
  composes them from its own env.lua, which adds its stubs and fixtures.
- Tests are declared with Test.test(name, fn) and every test function
  receives a fresh environment, so nothing carries over between them. A test
  which errors is reported as a failure rather than aborting the file.
- Every stub raises on properties it does not implement, which mirrors the
  game api. game.player is the one property allowed to read as nil.
- Test.deep_eq compares tables recursively with keys checked from both sides.
- The message records round trip through the same testMatrix and
  testRoundTripJsonSerialisable helpers the clusterio tests use, covering
  every optional field combination of the records, events and requests.
- controller.test.js and instance.test.js cover the node side of the plugin
  against faked controller and instance internals: property creation and
  sweeping, record building, broadcasts, subscription replay, assignment
  validation, auto assignment and its blocking role, seeding, the initialise
  payload, sync mode gating, and the rejection rollback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/common.js

@@ -0,0 +1,41 @@
+"use strict";
+const assert = require("assert").strict;
+const { compile } = require("@clusterio/lib");
+
+/**
+ * Generate a flat array of tests from a matrix of inputs.
+ *
+ * @template {any[][]} T
+ * @param {T} arrays - A list of arrays representing the different values for each argument
+ * @returns {Array<{ [K in keyof T]: T[K][number] }>}
+ * 		An array of tuples, each containing one value from each input array.
+ */
+function testMatrix(...arrays) {
+	return arrays.reduce((acc, curr) => acc.flatMap(a => curr.map(b => [...a, b])), [[]]);
+}
+
+/**
+ * Test that a class is round trip json serialisable across multiple test cases.
+ *
+ * @template {any[]} T - Constructor arguments
+ * @param {{new(...args: T): object}} Class - The class which has toJSON and fromJSON methods.
+ * @param {T[]} tests - The tests inputs to pass to the class constructor.
+ */
+function testRoundTripJsonSerialisable(Class, tests) {
+	const validate = compile(Class.jsonSchema);
+	for (const test of tests) {
+		const original = new Class(...test);
+		const serialised = JSON.stringify(original);
+		const jsonObject = JSON.parse(serialised);
+		const reconstructed = Class.fromJSON(jsonObject);
+		assert.deepEqual(reconstructed, original, JSON.stringify(test));
+		if (!validate(jsonObject)) {
+			throw validate.errors;
+		}
+	}
+}
+
+module.exports = {
+	testMatrix,
+	testRoundTripJsonSerialisable,
+};