fix mask_image in test_brushnet.py

[xduzhangjiayu]

What does this PR do?

Hi, I have tested SD-1.5 with "examples\brushnet\test_brushnet.py", but i found that the results were not as same as the results in folder "examples\brushnet\src". I found that the reason was the mask_image was inverted, so i fixed it in this PR, now it can generate right output. I will be happy if this PR can be merged, also i think are the same logic with "examples\brushnet\app_brushnet.py" and "examples\brushnet\test_brushnet_sdxl.py", so please let me know if you need another PR to fix them, thanks~