Allow jest globals in __mocks__ directories (facebook#25738)

Summary:
The `__mocks__` directories should be treated the same as `__tests__`
for the purpose of linting. See https://jestjs.io/docs/en/manual-mocks

## Changelog

[Internal] [Changed] - eslint: allow jest globals in `__mocks__` directories
Pull Request resolved: facebook#25738

Test Plan:
I installed the updated package locally in my project and verified
that the `no-undef` rule no longer fires for references to `jest` in
`__mocks__` directories.

Differential Revision: D16515546

Pulled By: osdnk

fbshipit-source-id: ae78b95d33d1b48ffa4aafa7a53c83e152d176b2

Author: artdent

packages/eslint-config-react-native-community/index.js

@@ -58,7 +58,10 @@ module.exports = {
       },
     },
     {
-      files: ['*.{spec,test}.{js,ts,tsx}', '**/__tests__/**/*.{js,ts,tsx}'],
+      files: [
+        '*.{spec,test}.{js,ts,tsx}',
+        '**/__{mocks,tests}__/**/*.{js,ts,tsx}',
+      ],
       env: {
         jest: true,
         'jest/globals': true,