Asset basenames in Jest snapshots

Summary:
This change only affects tests run with Jest. `require('/images/image1.png')` will be replaced with

```
Object {
  "testUri": "relative/path/to/images/image1.png",
}
```

in the Jest snapshot instead of always being 1 returned by RelativeImageStub. This change makes it possible to test conditional asset loading in components.

The problem with this change is that it will probably break a lot of existing snapshots, but that should be easily fixed when a project updates to a new version of React Native by running `jest -u` to update all snapshots.

A component can have conditional asset loading based on its props, this logic would be nice to test with Jest snapshots. This problem has been discussed in https://github.com/facebook/jest/issues/2838.

* **Who does this affect**: Everyone using `Image` in Jest snapshots
* **How to migrate**: Running `jest -u` will update the snapshots, the snapshots should be reviewed that they are correct.
* **Why make this breaking change**: It enables testing of conditional asset loading.
* **Severity (number of people affected x effort)**: Low.
Closes https://github.com/facebook/react-native/pull/13319

Reviewed By: rafeca

Differential Revision: D5708180

Pulled By: mjesun

fbshipit-source-id: 16ac42004d597db08545a21d4fffe95c5ee7e21f