If you write a library and want to provide 1! useful test function, where do you store this function? In the main source with a comment, in the main source with an extra annotation like

TestingOnly

, or do you really create a new module containing only 1 function and publish it?

I'd favour a new module - you can use gradle's

testFixtures

configuration to keep it in the same gradle project as the main source. After all, it may be one now but there's a decent chance it grows. It may also have different dependency requirements to the main source.