Summary:
This try to address #17672 and https://github.com/facebook/metro/issues/139. We should probably not include these folders in the released version of React Native, as they are used only for testing—am I incorrect?
cc MoOx, t4deu.
I ran:
```
npm pack
tar -t -f react-native-1000.0.0.tgz | less
```
Then verified that `__fixture__` was not part of the package.
https://github.com/facebook/react-native/pull/17672
[GENERAL] [BUGFIX] [.npmignore] - Do not publish test-specific files
Closes https://github.com/facebook/react-native/pull/18019
Differential Revision: D7098211
Pulled By: jeanlauliac
fbshipit-source-id: 0748ad8c064450bd2a9f4d6f49675a7f74fb300f
Jean Lauliac