test(wasm): reference atob polyfill in browser test (#1179)

The "target environment browser" test invoked the bundled output, which
leads to an undefined error on node 14 as [`atob` was introduced in node
16][0].

This commit uses the same atob polyfill that was introduced in #334

[0]: https://developer.mozilla.org/en-US/docs/Web/API/atob#browser_compatibility

packages/wasm/test/fixtures/async.js

@@ -1,5 +1,8 @@
 import fixture from './sample.wasm';
 
+// eslint-disable-next-line no-undef
+globalThis.atob = (str) => Buffer.from(str, 'base64').toString('binary');
+
 // eslint-disable-next-line no-undef
 result = (async () => {
   const module = await fixture();