Add `setSelectorMappings` method to view fixture.

This allows users to create a shorthand for a selector, so that the same selector doesn't need to be repeated across different tests.

So you can do this:

```javascript
viewFixture.setSelectorMappings({'my-mapping': '.some .selector'});
```

And then in your test you can do:

```javascript
then("form.view.(my-mapping).text = 'foo'");
```

This helps with maintainability and makes the tests easier to read.

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