Enable unit test for wasi-nn WinML backend.

[jianjunz]

This test was disabled because GitHub Actions Windows Server image doesn't have desktop experience included. But it looks like we can have a standalone WinML binary downloaded from ONNX Runtime project.

Wasi-nn WinML backend and ONNX Runtime backend now share the same test code since they accept the same input, and they are expected to produce the same result. Pre-processing and post-processing are added to nn_image_classification_onnx for improving its accuracy.

This change also make wasi-nn WinML backend as a default feature as it's covered by test now.

Fixes #8391.