drm/xe/kunit: Drop custom struct platform_test_case

Custom struct platform_test_case definition in xe_wa is now almost
identical to generic struct xe_pci_fake_data defintiion except the
.name member, which could be generated by xe_pci_fake_data_desc().

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250829171922.572-5-michal.wajdeczko@intel.com