That's not a bug, but a feature. The idea is that reviewers see that a unittest is @system or @safe and can point out cases where the test should have been @safe.
Also in a diff you typically don't see the module-level attributes + unittests are by default @system and yes we enforce the annotation in order to raise awareness of annotating the unittests.
I see the the point to extend it for module-level attributes, but that's not how this check was intended to be. Hence, I am closing this as WONTFIX.
BTW
- module-level attributes are very seldomly used at Phobos because they make reviewing really tricky
- this is the wrong component, it's a DScanner check