Add tests for client aborted requests.
It would be a really awesome thing to have automated tests for this; because it is pretty tricky to handle right and the last thing we want is for GraphQL servers to crash every time an impatient user presses the browser's back button while waiting for a large upload.

We need to be able to abort at specific moments of the processing to test:

- `Upload` scalar promises reject with an `UploadPromiseDisconnectUploadError` when the request disconnects before their file upload stream has begun parsing.
- Resolved file upload streams emit the `error` event with `FileStreamDisconnectUploadError` when the request disconnects while the stream is being parsed.

It would be great to also clarify/test what happens when the request is aborted before the first multipart field ([`operations`](https://github.com/jaydenseric/graphql-multipart-request-spec#multipart-form-field-structure)) is finished parsing.

In the last year I (@AdamYee also helped) spent perhaps over a week's worth of effort trying various approaches to simulate client aborted uploads, and always gave up and tested it manually using [`apollo-upload-examples`](https://github.com/jaydenseric/apollo-upload-examples) with the network speed throttled down via the Chrome dev tools.