It most probably comes from super-sources in one if your dependencies.
The way to debug it is to set a breakpoint in CompilingClassLoader, line 1142, conditionally breaking when the package name / fully qualified class name starts with "java.", to be able to see what exact class is the culprit; then search for that class' sources in your dependencies.
And the workaround (proper way?) is to run your tests in web/prod mode.
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