Add libwebp test

data @ 39dfdf21

-Subproject commit 4fe872bd290fb13ffbb808f3855018c745fcda1d
+Subproject commit 39dfdf2129cffec578970eee38846d3e8c1652b2

wasm/tests/libwebp.test.js

+const fs = require('fs');
+const path = require('path');
+const { TIMEOUT } = require('./config');
+const { runFFmpeg } = require('./utils');
+const IN_FILE_NAME = 'image.png';
+const OUT_FILE_NAME = 'image.webp';
+const FILE_SIZE = 6376;
+let pngData = null;
+
+beforeAll(() => {
+  pngData = Uint8Array.from(fs.readFileSync(path.join(__dirname, 'data', IN_FILE_NAME)));
+});
+
+test('transcode png to webp', async () => {
+  const args = ['-i', IN_FILE_NAME, OUT_FILE_NAME];
+  const { fileSize } = await runFFmpeg(IN_FILE_NAME, pngData, args, OUT_FILE_NAME);
+  expect(fileSize).toBe(FILE_SIZE);
+}, TIMEOUT);