I decided to “cut the Gordian knot” and upload jme3-testdata to JFrog Bintray. That way, if anyone besides me wants to use it in a Gradle/Maven/Ivy build, it’s easily available:
repositories { maven { url "https://dl.bintray.com/stephengold/org.jmonkeyengine" } }
dependencies { compile 'org.jmonkeyengine:jme3-testdata:3.1.0-stable' }