[xnnpack] Refactor test/op_linear

Pull Request resolved: #7744

No functional change, just moving code around.
ghstack-source-id: 262422437
@exported-using-ghexport

Differential Revision: [D68350953](https://our.internmc.facebook.com/intern/diff/D68350953/)

Co-authored-by: Digant Desai <[email protected]>

Author: pytorchbot