How to include testing utils in the library

I have simple Java library (with Gradle) that is stored as an artifact on my private Nexus instance. The library provides custom serializer/deserializer used in the technology I'm using (details are not really important) - I'm using this library in many projects

Now the problem is that to write integration tests for classes using the library I need to provide some additional configuration for this wrapped serializer/deserializer to mock some additional mechanism provided by the technology in production environment (making long story short - there is additional 3rd party service and running this during integration tests would be completely inefficient thus it's better to mock this)

What I've done in this situation - I created some TestUtils class providing serializer/deserializer that is extending library's implementations with proper configuration. The problem is that I need to have a copy of this TestUtils in every project using the library which obviously is not the best idea because in case of any future changes in the library I will need to manually update TestUtils in every project that will use newer version of the library

I decided to somehow include TestUtils class into my library but now I have a problem how to do this in best way, maybe using some Gradle specific way of work that I'm not aware of

What I figure out is to either:

• put the TestUtils class to the library's test package and put it to the result artifact - seems to be bad idea as described e.g. here: Configure Gradle to include main classes as well as test classes into my .jar archive
• put the TestUtils class to the library's main package (create specific subpackage for this) - which seems for me to be awful idea at least because the developer could use this class accidentally in a production code
• create new library named e.g. my-library-test-utils - seems to be the most clear way of doing this but will enfore me to handle two artifacts independently and remember to bump up test-utils every time the library will be updated
• just leave TestUtils as it is
• use some Gradle's magic to keep library's sources and TestUtils together then export it to two artifacts or make something else like exporting fatJar with test-utils inside accesssible from outside, or make TestUtils class available only during tests (is this even possible?)