Add scripts for running tests locally

Summary:
I was going to add an integration test and realized it would be useful to have an easy way to run these test locally. Added scripts and documented how to use them.

**Test plan**

`./scripts/run-android-local-unit-tests.sh`

<img width="786" alt="screen shot 2016-05-04 at 3 55 26 pm" src="https://cloud.githubusercontent.com/assets/346214/15018667/7f4981cc-1212-11e6-9fcb-12493c29015c.png">

`./scripts/run-android-local-integration-tests.sh`

<img width="772" alt="screen shot 2016-05-04 at 3 57 23 pm" src="https://cloud.githubusercontent.com/assets/346214/15018677/90b54810-1212-11e6-83d4-58530eb41d79.png">

Buck check by replacing `which buck` by `which duck` in the scripts:

<img width="805" alt="screen shot 2016-05-04 at 4 09 37 pm" src="https://cloud.githubusercontent.com/assets/346214/15018696/aa008262-1212-11e6-9a22-173507cd771f.png">

Checked the website renders fine:

`cd website; npm install; npm start`

![screen shot 2016-05-04 at 4 05 23 pm](https://cloud.githubusercontent.com/asse
Closes https://github.com/facebook/react-native/pull/7386

Differential Revision: D3258717

fb-gh-sync-id: 023eb9fdb59b00f9507e2e0694ad0934bb564c03
fbshipit-source-id: 023eb9fdb59b00f9507e2e0694ad0934bb564c03

docs/AndroidBuildingFromSource.md

@@ -129,6 +129,10 @@ gradle.projectsLoaded {
 }
 ```
 
+## Testing
+
+If you made changes to React Native and submit a pull request, all tests will run on your pull request automatically. To run the tests locally, see [Testing](/react-native/docs/testing.html).
+
 ## Troubleshooting
 
 Gradle build fails in `ndk-build`. See the section about `local.properties` file above.