Remove test folder from dawn source tree (#27247)

### Description  
Remove the test folder from the dawn source tree to potential file scan
operations by eliminating unnecessary files.

The `test` folder contains 70k+ files, and we don't need them in ONNX
Runtime build at all.

Author: fs-eire

cmake/external/onnxruntime_external_deps.cmake

@@ -764,7 +764,11 @@ if (onnxruntime_USE_WEBGPU)
           # - (private) Fix compatibility issues with Safari. Contains the following changes:
           #   - Polyfill for `device.AdapterInfo` (returns `undefined` in Safari v26.0)
           #
-          ${Patch_EXECUTABLE} --binary --ignore-whitespace -p1 < ${PROJECT_SOURCE_DIR}/patches/dawn/safari_polyfill.patch)
+          ${Patch_EXECUTABLE} --binary --ignore-whitespace -p1 < ${PROJECT_SOURCE_DIR}/patches/dawn/safari_polyfill.patch &&
+
+          # Remove the test folder to speed up potential file scan operations (70k+ files not needed for build).
+          # Using <SOURCE_DIR> token ensures the correct absolute path regardless of working directory.
+          ${CMAKE_COMMAND} -E rm -rf <SOURCE_DIR>/test)
 
       onnxruntime_fetchcontent_declare(
         dawn