ci: implement screenshot diffing (#2571)

tests/reftest-diff.ts

@@ -0,0 +1,32 @@
+import {sync} from 'glob';
+import {resolve, basename} from 'path';
+import {existsSync, promises} from 'fs';
+import {toMatchImageSnapshot} from 'jest-image-snapshot';
+
+const resultsDir = resolve(__dirname, '../results');
+const customSnapshotsDir = resolve(__dirname, '../tmp/snapshots');
+const customDiffDir = resolve(__dirname, '../tmp/snapshot-diffs');
+
+expect.extend({toMatchImageSnapshot});
+
+describe('Image diff', () => {
+    const files: string[] = sync('../tmp/reftests/**/*.png', {
+        cwd: __dirname,
+        root: resolve(__dirname, '../../')
+    }).filter((path) => existsSync(resolve(resultsDir, basename(path))));
+
+    it.each(files.map((path) => basename(path)))('%s', async (filename) => {
+        const previous = resolve(resultsDir, filename);
+        const previousSnap = resolve(customSnapshotsDir, `${filename}-snap.png`);
+        await promises.copyFile(previous, previousSnap);
+        const updated = resolve(__dirname, '../tmp/reftests/', filename);
+        const buffer = await promises.readFile(updated);
+
+        // @ts-ignore
+        expect(buffer).toMatchImageSnapshot({
+            customSnapshotsDir,
+            customSnapshotIdentifier: () => filename,
+            customDiffDir
+        });
+    });
+});