message: chore(build): give each bundling test its own tmpdir
  This *might* fix flakiness like this:
  [2021/04/19 11:53:57.707]   1) compile generateBundle
  [2021/04/19 11:53:57.707]        does not attempt to load ES6 modules because parcel cannot handle them properly:
  [2021/04/19 11:53:57.707]      Error: ENOENT: no such file or directory, open 'Z:\data\mci\b2b62ed7d54f2bcd3c5e2ddd5846c7d9\src\packages\tmp\test-build-1618833168859-0.6650505642822344\a.js'
diff: @@ -10,11 +10,12 @@ const execFile = promisify(childProcess.execFile);
  describe('compile generateBundle', function() {
  this.timeout(60_000);
  - const tmpdir = path.resolve(
  - __dirname, '..', '..', '..', 'tmp', `test-build-${Date.now()}-${Math.random()}`
  - );
  + let tmpdir: string;
  beforeEach(async() => {
  + tmpdir = path.resolve(
  + __dirname, '..', '..', '..', 'tmp', `test-build-${Date.now()}-${Math.random()}`
  + );
  await fs.mkdir(tmpdir, { recursive: true });
  });
  