Pablo,
Sorry, I missed an important point.
You will have to copy that file, CoreAuthenticationTestUtils, into your project (src/test/java/...). And update it during upgrades.
The test classes are not part of the jar files; just the files under src/main/java/...
When you build, if the included file has dependencies, you will get errors. Just repeat the steps to find the missing file and package.
The test classes that you might include should 'not' depend on too many other test classes, just the main classes (hopefully). I have only 2. If you find that you are copying in many test classes, then perhaps the feature you are working on would be better
suited to inclusion in the cas project,
https://fawnoos.com/2020/10/22/cas63x-codebase-feature-build/
Ray
P.S. This is my list of library includes (some may not be needed)
testCompileOnly "org.apereo.cas:cas-server-core-webflow-api"
testCompileOnly "org.apereo.cas:cas-server-core-util-api"
testImplementation "org.apereo.cas:cas-server-core-services-authentication"
testImplementation "org.apereo.cas:cas-server-core-authentication-api"
testImplementation "org.apereo.cas:cas-server-core-services"
testImplementation "org.apereo.cas:cas-server-core"
testImplementation "org.apereo.cas:cas-server-core-api"
testImplementation "org.apereo.cas:cas-server-support-person-directory"