If Test Fragment holds only pre processor/timer, Due to scoping rules, elements won't be executed and therefore it's useless and Module and Include Controller should not allow to add it. Following a question https://stackoverflow.com/questions/48078118/test-fragment-not-executing-timers-or-sample-timeout and related issue 61965
In relation to Bug 61965
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/4685