要約

beforeEach() は常に Promise が (then() まで含め) 全部満たされてから test() に移る。
setTimeout() は呼ばれない。

実行例

test.js

describe('async test', () => {
  let called;

  function promise() {
    console.log(called = 'function');

    setTimeout(() => {
      console.log(called = 'setTimeout()');
    }, 0);

    return new Promise(resolve => {
      console.log(called = 'Promise()');
      resolve();
    }).then(() => {
      console.log(called = 'then()');
    });
  }

  describe('test() only', () => {
    test('without await', () => {
      promise();
      expect(called).toBe('Promise()');
    });
  });

  describe('test() only', () => {
    test('with await', async () => {
      await promise();
      expect(called).toBe('then()');
    });
  });

  describe('beforeEach() and test()', () => {
    beforeEach(() => {
      promise();
    });

    test('without await', () => {
      expect(called).toBe('then()');
    });
  });

  describe('beforeEach() and test()', () => {
    beforeEach(async () => {
      await promise();
    });

    test('with await', () => {
      expect(called).toBe('then()');
    });
  });
});

yarn run v1.22.10
$ jest --verbose
 PASS  ./test.js
  async test
    test() only
      ✓ without await (12 ms)
      ✓ with await (2 ms)
    beforeEach() and test()
      ✓ without await (1 ms)
      ✓ with await (7 ms)

  console.log
    function

      at promise (test.js:5:13)

  console.log
    Promise()

      at Promise.then.console.log.called (test.js:12:15)

  console.log
    then()

      at test.js:15:15

  console.log
    function

      at promise (test.js:5:13)

  console.log
    Promise()

      at Promise.then.console.log.called (test.js:12:15)

  console.log
    then()

      at test.js:15:15

  console.log
    function

      at promise (test.js:5:13)

  console.log
    Promise()

      at Promise.then.console.log.called (test.js:12:15)

  console.log
    then()

      at test.js:15:15

  console.log
    function

      at promise (test.js:5:13)

  console.log
    Promise()

      at Promise.then.console.log.called (test.js:12:15)

  console.log
    then()

      at test.js:15:15

Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total
Snapshots:   0 total
Time:        0.979 s, estimated 1 s
Ran all test suites.
✨  Done in 1.83s.

参考

Microtasks