This allows to group together related AST classes to reuse the same
test source and extraction. For example this is useful for
`EnumDecl/EnumCaseDecl/EnumElementDecl`, where this is applied to.
This was temporarily broken as we were skipping full emission of all
entities without any valid location.
We now rely on `decl->getDeclContext()->getParentSourceFile()` which is
more robust.