rewrite mapreduce upsert tests with async/await

[emilygilberts]

• rewrote the unit tests for mapreduce.js-upsert using async/await
  (left the mapreduce.js-utils tests as they are, because they are more readable & straightforward with the current syntax)

[janl]

this looks good overall, I’m just wondering if it would make sense to DRY out the get and put functions a little bit, it’s three lines each when the only difference is the what the promise resolves with.

Spitballing here:

instead of

   const res = await upsert(
      {
        get: function () {
          return Promise.reject({ status: 404 });
        },
        put: function () {
          return Promise.resolve({ rev: 'abc' });
        },
      },
      'foo',
      function () {
        return { difference: "something" };
      },
    );

function makeFun (action, value) {
  return function() { return Promise[action](value) };
}

// [...]

    const res = await upsert(
      {
        get: makeFun('resolve', {status: 404}),
        put: makeFun('reject', { rev: 'abc' }),
      },
      'foo',
      function () { return { difference: "something" };
      },
    );

Might be too fancy, feel free to shoot me down :)

[janl]

just a style nit, but I’d be okay with having the opening curlies start with the method: upsert({ to save a mostly empty line