Promise returning matchers are not awaited if matcher has no async keyword.

[hiyelbaz]

Describe the bug

This breaks jest compatibility and prevents migrating some matcher extension libraries to vitest. Detailed example is below.

Reproduction

Vitest StackBlitz link: https://stackblitz.com/edit/vitest-dev-vitest-hwzkyd?file=test%2Fasync.test.js&initialPath=__vitest__
Jest StackBlitz link: https://stackblitz.com/edit/jest-example-kymfnr?file=async.test.js&terminal=test&view=editor

import { it, expect } from 'vitest';

//gets the value after 100ms asynchroniously
async function getAsyncResult() {
  return new Promise((resolve) => {
    setTimeout(() => resolve('expected result'), 100);
  });
}

// this matcher function has 'async' keyword and returns promise
async function toBeExpected_Original(text) {
  const { isNot } = this;
  const value = await getAsyncResult();
  const result = value === text;
  return {
    pass: isNot ? !result : result,
    message: () => `Input is ${result ? '' : 'not '}'expected result'`,
  };
}

// this matcher is the same function with toBeExpected_NonAsync but has 'async' keyword
async function toBeExpected_Async(text) {
  // ... process text before using the original matcher for some reason...
  // reuse the original matcher
  return toBeExpected_Original.call(this, text);
}

// this matcher function has no 'async' keyword but returns the promise from original matcher.
function toBeExpected_NonAsync(text) {
  // ... process text before using the original matcher for some reason...
  // reuse the original matcher
  return toBeExpected_Original.call(this, text);
}

const extensionMatchers = {
  toBeExpected_Original,
  toBeExpected_Async,
  toBeExpected_NonAsync,
};

expect.extend(extensionMatchers);

// OK
it('async matcher works as expected.', async () => {
  await expect('expected result').toBeExpected_Original();
});

// OK
it('async matcher that uses existing matcher works as expected.', async () => {
  await expect('expected result').toBeExpected_Async();
});

// NOT OK
it('sync but promise returning matcher fails.', async () => {
  await expect('expected result').toBeExpected_NonAsync();
});

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (16) x64 AMD Ryzen 7 PRO 4750U with Radeon Graphics
    Memory: 40.90 GB / 63.37 GB
  npmPackages:
    @vitejs/plugin-react: ^3.0.1 => 3.0.1
    vite: ^4.0.4 => 4.0.4
    vitest: ^0.27.1 => 0.27.1

Used Package Manager

npm

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