When running `MockedProvider` tests from a CLI or CI, those
tests are run using the CommonJS bundled version of React Apollo
(`react-apollo.cjs.js`). The `test-utils` themselves however, like
`MockedProvider`, are not included in the production
`react-apollo.cjs.js` bundle, to help keep the prod bundle size
down. To use `MockedProvider`, it must be accessed using a
nested/direct import like:

```
import { MockedProvider } from "react-apollo/test-utils";
```

Unfortunately, this nested import means `MockedProvider` is
loaded outside of the `react-apollo.cjs.js` bundle, which means
it requires and uses its own versions of other supporting
React Apollo files. This causes problems in areas like using
React's Context API. Both `MockedProvider` and the
component's it's attempting to test create and use their own
versions of shared Context, which are not available to each
other. So when `MockedProvider` creates a new Context and adds
the `client` instance to it, its child components don't have
access to it, since they've created their own Context
separately.

In React Apollo 3.0 we're going to move the `test-utils` out
into their own package, which will fix this problem. As a
workaround for now, this commit forces the `test-utils` to use
the `react-apollo.cjs.js` version of React Apollo. This means
tests have access to the same dependencies as the components
being tested.

Fixes #2900.