Cleanup: Move vulkan specific tests in tests folder

When reviewing the render graph core PR, we discussed where specific
backend tests should be located. The code style is clear that it
needs to be located in a tests folder next to the code it tests.

This PR moved the tests folder from next to the files they test to
a tests folder.

Pull Request: https://projects.blender.org/blender/blender/pulls/120777

source/blender/gpu/CMakeLists.txt

@@ -910,8 +910,8 @@ if(WITH_GTESTS)
 
   if(WITH_VULKAN_BACKEND)
     list(APPEND TEST_SRC
-      vulkan/vk_data_conversion_test.cc
-      vulkan/vk_memory_layout_test.cc
+      vulkan/tests/vk_data_conversion_test.cc
+      vulkan/tests/vk_memory_layout_test.cc
     )
   endif()