And a thorough description of the problem / question:
If you define reusable classes for unit testing following gradle standards in ‘testFixtures’ and those classes make use of junit assertions (e.g., org.junit.jupiter.api.Assertions.assertTrue), Sonarlint flags a bug that “Assertions should not be used in production code”.
It seems to me that in this case, you are reaching the current limit of our rule classification. Rule java:S5960 is explicitly trying to identify the wrong use of assertions in production code… Buch such calls are always going to be valid when designing a testing frameworks.
Unfortunately, there is no way to let our analyzer know that it is analyzing a project aiming at tests… and so this rule is non-sensical in this context. I would encourage you to ignore the issues and mark them as won’t fix (or disable the rule).