CodeGen(vk_parser): Unit Test Case Fixes

Key Updates:
- Fixed unit test cases in test_vkjson_gen_util.py following vkjson_generator refactoring.
- Replaced 'VK_NV_ray_tracing' with 'VK_FUCHSIA_external_semaphore' as VK_NV is added to known prefix map.
- Fixed tests using 'generate_vk_version_structs_initialization' and 'generate_vk_extension_structs_init_code' as function parameters have changed.
- Added new mock variables like 'STRUCT_EXTENDS_MAPPING' and 'ADDITIONAL_EXTENSION_INDEPENDENT_STRUCTS' as they are required by 'TestGenerateVkCoreStructsInitCode' and 'TestGenerateMemsetStatements'

Test: adb shell cmd gpu vkjson
Flag: NONE infeasible
Bug: b/401184058
Change-Id: I874ef11e2a77eab08c200658725889a0a1c755c4