assert.throwsAsync incorrectly tolerates a result then which returns a non-thenable

[gibson042]

e.g.,

asyncTest(async function() {
  const badThenable = { then(){} };
  assert.throwsAsync(Error, function() { return badThenable; });
});

should not succeed (because the thenable never settles to anything, let alone the expected rejection), but it does because the result of then is used to directly resolve the assert.throwsAsync promise (i.e., assuming that then is itself conforming). Instead, res should itself be absorbed into an actual Promise which will definitely invoke the appropriate then callback upon settlement.