Create how-to-test.md

Author: Meaghan Lewis

docs/contributing/how-to-test.md

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+Unit and Integration tests for Unity can be found under `src/tests/`.
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+## Testing requirements
+Unit and integration tests currently run with NUnit 2.6.4.
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+## Running tests
+Tests can be run after building the Unity project. To run the tests execute `test.cmd` on Windows or `test.sh` on Mac.
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+We use [Appveyor](https://ci.appveyor.com/project/github-windows/unity/build/tests) as the CI for this project to run tests, but it is also necessary to run tests locally when making code changes.