Add test environment to readme (#649)

Author: jan-janssen

README.md

@@ -157,6 +157,7 @@ as hierarchical job scheduler within the allocations.
   * [License](https://executorlib.readthedocs.io/en/latest/4-developer.html#license)
   * [Modules](https://executorlib.readthedocs.io/en/latest/4-developer.html#modules)
   * [Interface Class Hierarchy](https://executorlib.readthedocs.io/en/latest/4-developer.html#interface-class-hierarchy)
+  * [Test Environment](https://executorlib.readthedocs.io/en/latest/4-developer.html#test-environment)
   * [Communication](https://executorlib.readthedocs.io/en/latest/4-developer.html#communication)
   * [External Libraries](https://executorlib.readthedocs.io/en/latest/4-developer.html#external-libraries)
   * [External Executables](https://executorlib.readthedocs.io/en/latest/4-developer.html#external-executables)