Import punet tests from sharktank to iree_tests.

[ScottTodd]

Related to https://github.com/nod-ai/sharktank/issues/76

• Model source: https://github.com/nod-ai/sharktank/tree/main/sharktank/sharktank/models/punet
• Python test: https://github.com/nod-ai/sharktank/blob/main/sharktank/integration/models/punet/integration_test.py
• Python run script: https://github.com/nod-ai/sharktank/blob/main/sharktank/sharktank/models/punet/tools/run_punet.py
• Weights are labeled in the sharktank sources. The primarily come from https://huggingface.co/stabilityai/stable-diffusion-xl-base-1.0 and https://huggingface.co/amd-shark/sdxl-quant-models

Only testing compilation and basic execution for now. Result comparisons will follow once they are enabled upstream. TBD if the quantized outputs will be different enough across backends to need extra infrastructure configuration there.

[ScottTodd]

This will have conflicts with https://github.com/nod-ai/SHARK-TestSuite/pull/280, but otherwise it should be ready to land.