Tests: Added README

test/unit/README.md

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+## Setup
+
+- Install [Node.js](https://nodejs.org/)
+- Execute `npm install`
+
+## Build
+
+- Execute `npm run build-test`
+
+## Run
+
+You can run the unit tests in two environments:
+
+- Node.js: Execute `npm run test`
+- Browser: Execute `npm start` and call `http://localhost:8080/test/unit/UnitTests.html` (see [How to run things locally](https://threejs.org/docs/#manual/introduction/How-to-run-things-locally))