The Groovy tests in nifi-properties-loader were not running because there was no src/test/java directory. This is a known issue. The two solutions (in preferred order):
1. Add a custom plugin configuration to the module's pom.xml to explicitly add the src/test/groovy directory as a resource
1. Add an empty file in src/test/java/Example.java
The latter is confusing to developers unfamiliar with this issue and subject to accidental "cleanup" in the future.