test: add documentation with `TESTING.md`

docs/TESTING.md

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+# Testing PlantUML
+
+## Prerequisites
+
+See the [Building page](../BUILDING.md) and especilay the paragraph [Building PlantUML with Gradle](../BUILDING.md#building-plantuml-with-gradle).
+
+## Running Tests
+
+### Run all tests, for all licences
+
+To run the tests included with the project, use the following command:
+
+```sh
+gradle test
+```
+
+### Run a specific test, for only one licence
+
+Comment those lines on [`settings.gradle.kts`](../settings.gradle.kts):
+
+https://github.com/plantuml/plantuml/blob/a327d636a7fcc7f05a88b796a2838da16e2ba3e3/settings.gradle.kts#L12-L16
+
+Then you can run a specific test _(e.g. the `aTestClass` Class)_:
+```sh
+gradle test --tests aTestClass
+```
+
+## Contributing
+
+After successfully building and testing the project, you are ready to start [contributing](../CONTRIBUTING.md) to PlantUML!
+If you have any changes to contribute, please submit a pull request through the [PlantUML GitHub repository](https://github.com/plantuml/plantuml).
+
+
+## Test Directory Architecture
+
+- [test/](../test)
+  - [com/plantuml/wasm](../test/com/plantuml/wasm)
+  - [net/sourceforge/plantuml](../test/net/sourceforge/plantuml)
+  - [nonreg](../test/nonreg)
+
+```mermaid
+%%{ init : { "flowchart" : { "curve" : "stepBefore" }}}%%
+graph LR
+T["test/"]
+T -->|"Unit Test\n(of src/com/plantuml/api/cheerpj)"| W["com/plantuml/wasm"]
+T -->|"Unit Test\n(of src/net/sourceforge/plantuml)"| U["net/sourceforge/plantuml"]
+T -->|"Non-regression Test"| N[nonreg]
+```
+
+## Additional Resources
+
+### Gradle
+- site: [Gradle](https://gradle.org)
+- doc: [Gradle User Guide](https://docs.gradle.org/current/userguide/userguide.html)
+- src: [:octocat:Gradle](https://github.com/gradle/gradle) 
+
+### Gradle Plugins
+- site: [Gradle Plugins Search](https://plugins.gradle.org)
+- A "gradle test logger plugin"
+  - src: [:octocat:radarsh/gradle-test-logger-plugin](https://github.com/radarsh/gradle-test-logger-plugin)
+
+### JUnit
+- site: [JUnit5](https://junit.org/junit5/)
+- doc: [JUnit User Guide](https://junit.org/junit5/docs/current/user-guide/)
+- src: [:octocat:JUnit-team](https://github.com/junit-team)
+
+### JUnit extensions
+- doc: [JUnit extensions](https://junit.org/junit5/docs/current/user-guide/#extensions)
+- The "Glytching" JUnit extensions
+  - doc: [Glytching JUnit extensions](https://glytching.github.io/junit-extensions/)
+  - doc: [`randomBeans` doc](https://glytching.github.io/junit-extensions/randomBeans)
+  - src: [:octocat:glytching/junit-extensions](https://github.com/glytching/junit-extensions)
+
+### AssertJ
+- doc: [AssertJ](https://assertj.github.io/doc/)
+- src: [:octocat:AssertJ](https://github.com/assertj/assertj)
+
+### Mockito
+- site: [Mockito](https://site.mockito.org)
+- doc: [Mockito doc.](https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html)
+- src: [:octocat:Mockito](https://github.com/mockito/mockito)