tests: remove XHRMock util

[peaBerberian]

This is a Work-In-Progress to remove the XHRMock utils from our tests, which monkey-patched the browser's XMLHttpRequest API to allow mocking requests, "locking" them, redirecting them and so on (kind of like sinon.js's nise which was a major inspiration but with added locking/unlocking capabilities).

The reasons for doing this are:

• It only worked for XMLHttpRequest and not for the fetch browser API which we use to fetch low-latency segments for now (due to some API differences between the two) and plan to use for more things in the future.

  Adding support for fetch looks like huge work (we have to re-implement the most part of the API in JS - may be fun but there are funnier tasks which are also more useful :p)

• We have RxPlayer API which allows an application to provide its own fetching logic (manifestLoader and segmentLoader) which covers almost all fetching cases (it does not cover DASH's XLink loading, low-latency segment fetching and future Content Steering Manifest fetching) and covers all fetching cases currently done by XHRMock anyway.

• The now probable incoming future in-Worker feature of the RxPlayer cannot easily rely on monkey-patching native API it uses, at least in integration tests (well, in all honesty, we will have to think about how we'll be testing this).

• It was complex, looked like an NIH-y fakeServer implementation.

  Removing it in favor or simpler solutions is just easier to understand.

This is not completely done yet.