Why am I getting "Error: Resolution method is overspecified"?

Just drop
.then(done); and replace function(done) with function()

You are returning a Promise so calling done is redundant as it said in error message

In the elder versions you had to use callback in case of async methods like that

it ('returns async', function(done) {
   callAsync()
   .then(function(result) {
      assert.ok(result);
      done();
   });
})

Now you have an alternative of returning a Promise

it ('returns async', function() {
  return new Promise(function (resolve) {
     callAsync()
       .then(function(result) {
          assert.ok(result);
          resolve();
       });
  });
})

But using both is misleading (see for example here https://github.com/mochajs/mocha/issues/2407)

Mocha allows to either use a callback:

it('should complete this test', function (done) {
  new Promise(function (resolve) {
    assert.ok(true);
    resolve();
   })
  .then(done);
});

OR return a promise:

it('should complete this test', function () {
  return new Promise(function (resolve) {
    assert.ok(true);
    resolve();
   });
});

// Or in the async manner
it('should complete this test', async () => {
    await Promise.resolve();
    assert.ok(true);
});

You can't do both.