Improve test-support

When working on #620 my new discovered some shortcomings of the current test-support. This is a spin-off of the mentioned PR to discuss ideas to improve it :)

## Utilize Test-Support ;)

Currently, the test-support for mirage is part of the public API for production code. It should be moved `test-support` to not pollute prod builds with that chunk of code.

Specifically: move `addon/mirage` to somewhere into `addon-test-support`

## Multiple Adapters

At the moment, `ember-file-upload` ships with a handler for mirage. Would be nice to have support for polly.js, too.

Even better, I think equally imporant are the test-support utilities, such as `extractFormData()` and `extractFileMetadata()`. They are very much suitable for integration tests, ie.

```
// understand this as pseudo-code, I'm not writing this from memory (API and names are likely to be different)
test('it uploads (with polly)', async function(assert) {
this.server.polly.post('/photos/new', (req, res) => {
extractFileMetadata(req.responseBody);
});
});
```

As I created those tests myself without/prior to knowing those utils exists, they are likely to work with either polly as well as mirage :)

## Imperatively Control the Upload Progress

Writing UIs around upload is definitely part of userland code. Giving developers a good way to test their UIs is key. To test what happens in each `file.state`? Using `await selectFiles()` would skip the `uploading` state and would require devs to either `waitFor()` of some dom node to appear or set some timeout on the request itself. Both are not reliable and have high potential to cause flaky tests, [read more in this discussion](https://github.com/adopted-ember-addons/ember-file-upload/pull/620#discussion_r783994808).

An idea would be to have an imperative API to programmatically advance the `state` and be _in control_. Basically, a deferred promise - this is _shim_ code:

```
test('it uploads (with polly)', async function(assert) {
const uploadHandler = new PollyUploadHandler();
this.server.polly.post('/photos/new', async (req, res) => {
return uploadHandler.handle((req, res) => {
// business logic code
});
});

await render(hbs`...`);

const upload = selectFilesForHandler('input[type="file"]', new File(...));

uploadHandler.startUpload(upload); // moves `state` into `uploading`

assert.dom('...').hasText('uploading...'); // asserts UI state for uploading

uploadHandler.success((req, res) => {
// possible business logic code
});

// --- OR ---

uploadHandler.fail(); // example for not making use of business logic

assert.dom('...').hasText('upload failed'); // assert UI for `error` state
});
```

This is a _rough_ idea but I myself have too many questions how to shape the API around that, when and where to allow business logic, etc. My hope is to get the discussion going.