test: add tests for addHelper priority values

This new test verifies that helpers are used in the correct order, corresponding to the priority
value provided to `addHelper`.

Author: Christopher Willis-Ford

test/unit/helper-priority.js

@@ -0,0 +1,50 @@
+const test = require('tap').test;
+
+const ScratchStorage = require('../../dist/node/scratch-storage');
+
+let storage;
+test('constructor', t => {
+    storage = new ScratchStorage();
+    t.type(storage, ScratchStorage);
+    t.end();
+});
+
+const logEntries = [];
+class FakeHelper {
+    constructor (label, shouldSucceed) {
+        this.label = label;
+        this.shouldSucceed = shouldSucceed;
+    }
+
+    /**
+     * Fetch an asset but don't process dependencies.
+     * @param {AssetType} assetType - The type of asset to fetch.
+     * @param {string} assetId - The ID of the asset to fetch: a project ID, MD5, etc.
+     * @param {DataFormat} dataFormat - The file format / file extension of the asset to fetch: PNG, JPG, etc.
+     * @return {Promise.<Asset>} A promise for the contents of the asset.
+     */
+    load (assetType, assetId, dataFormat) {
+        logEntries.push(this.label);
+        return this.shouldSucceed ?
+            Promise.resolve(new storage.Asset(assetType, assetId, dataFormat, Buffer.from(this.label))) :
+            Promise.reject(`This is an expected failure from ${this.label}`);
+    }
+}
+
+test('addHelper', t => {
+    storage.addHelper(new FakeHelper('third', true), -50);
+    storage.addHelper(new FakeHelper('first', false), 50);
+    storage.addHelper(new FakeHelper('second', false), 0);
+    t.end();
+});
+
+test('priorities', t => {
+    t.deepEqual(logEntries, []);
+    return storage.load('fakeAssetType', 'fakeAssetId', 'fakeDataFormat').then(() => {
+        t.deepEqual(logEntries, [
+            'first',
+            'second',
+            'third'
+        ]);
+    });
+});