add .gitignore for rn-tester build artifacts (#44041)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44041

changelog: [internal]

Building rn-tester left artifacts that would be picked up by mercurial.

```
❯ hg s
? rn-tester.xcodeproj.local/Project Settings.plist
? rn-tester.xcodeproj.local/Project Settings.plist.lock
```

To mitigate this, add  `rn-tester.xcodeproj.local` introduce .gitignore for rn-tester.

Reviewed By: fabriziocucci

Differential Revision: D56006193

fbshipit-source-id: 5701f1adf395e98f84ca59574dbd8747cf7e85db

packages/rn-tester/.gitignore

@@ -0,0 +1 @@
+rn-tester.xcodeproj.local/