Fix TestAssistants HeaderGuardAssistant tests

- after 0698385b we now need a use of an unguarded header
  to get the warning
1 job for !100 with test_assistant_fix in 51 minutes and 53 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Build
passed #30467
linux kf5-qt5 qt5.12

00:51:53

47.0%