When I ran into this issue back in 2017, I took the liberty of uploading the JAR to BinTray, so I can do:
<dependency>
<groupId>org.jmonkeyengine</groupId>
<artifactId>jme3-testdata</artifactId>
<version>3.1.0-stable</version>
<type>pom</type>
</dependency>
The repo location is
https://dl.bintray.com/stephengold/org.jmonkeyengine
Full info at