npmignore: ignore tests and fixtures

Summary:
This try to address #17672 and facebook/metro#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.

facebook/react-native#17672

[GENERAL] [BUGFIX] [.npmignore] - Do not publish test-specific files
Closes facebook/react-native#18019

Differential Revision: D7098211

Pulled By: jeanlauliac

fbshipit-source-id: 0748ad8c064450bd2a9f4d6f49675a7f74fb300f

.npmignore

@@ -0,0 +1,2 @@
+__fixtures__
+__tests__