Updating all references and docs on the `React.addons.TestUtils` and the
shallow renderer to refer to the correct targets.

Instead of:
```
const React = require('react');

// ...
React.addons.Testutils
// or

const ReactTestUtils = require('react-addons-test-utils');
```
we now show:
```
const ReactTestUtils = require('react-dom/test-utils');
```

And for shallow renderer, instead of:
```
const shallowRenderer = TestUtils.createRenderer();
```

we now show:
```
const shallowRenderer = require('react-test-renderer/shallow');
```

These flags are used to set arrow links to easily navigate through the
documents. They were wrong or missing in some of the 'add-ons' pages and
this bothered me when manually testing the updates from the previous
commit.

Missed this when updating the docs for the changes to shallow-renderer
in React 15.5.

Thanks @bvaughn for catching this

We should show using the same variable names between code samples.

We should use the same variable name for the same thing across examples.
`renderer` -> `shallowRenderer`.