Publish testing helpers

[asm89]

The library ships with some testing helpers. E.g. the test scheduler, but also a base test case and some helpers that are useful. We could move those to the lib/ (or src/ for 2.x) directory and make them useable by downstream users without having to rely on autoloader hacks.

More concretely:

• https://github.com/ReactiveX/RxPHP/blob/7aeb8cef74b29bdaa09ea969daf78577299a2aad/test/helper-functions.php
• https://github.com/ReactiveX/RxPHP/blob/7aeb8cef74b29bdaa09ea969daf78577299a2aad/test/Rx/Functional/FunctionalTestCase.php

[davidwdan]

You can autoload these with composer:

  "autoload-dev": {
    "psr-4": {
      "Rx\\": "vendor/reactivex/rxphp/test/Rx"
    },
    "files": [
      "vendor/reactivex/rxphp/test/helper-functions.php"
    ]
  },

But it would still be good to move them over to Testing within the Rx namespace.