How to test compiled JAR file using gradle

I had a code that was working correctly when was executed during standard unit testing, but didn't work when it was compiled into the jar and was added as dependency for some other project.

It wasn't an issue to find the root cause and fix it, but I started to think how can I test freshly made jar artifact before deploying it anywhere, to make sure that it will work for end users and other projects. I have googled this topic for several hours, but didn't even find something close to it.

Maybe I'm totally wrong and trying to achieve something weird, but I cannot figure out another way how to verify compiled packages and be confident that it will work for others.

Some details about the project - simple Java library with few classes, using Gradle 5.5 as a build system and travis-ci as CI/CD tool, for testing I'm using TestNG, but I can easily switch to JUnit if it will be required.

If you curious about the code, which was not working when was compiled into the package, here is simplified version:

public String readResourceByURI() throws IOException, URISyntaxException
{
  new String(Files.readAllBytes(Paths.get(ClassLoader.getSystemClassLoader().getResource("resource.txt").toURI())));
}

This function will throw java.nio.file.FileSystemNotFoundException if packaged into the jar file. But as I said the problem is not with the code...

Ideally, I want to create a build pipeline, that will produce jar artifacts, which then will be tested and if tests are successful those jars will be automatically deployed to repository (maven and/or bintray).

At the moment all tests are executed before jar creation and as a result there is chance, that compiled code inside jar package will not work due to packaging.