[CI][tests] Fix LTX-2.5 Diffuson Decoder Tiling CI CPU Timeout

[dg845]

What does this PR do?

The LTX-2.5 diffusion decoder TestLTX2VideoDiffusionDecoderModelTiling.test_tiles_covering_the_video_match_untiled_exactly test currently times out on the CI on CPU (https://github.com/huggingface/diffusers/actions/runs/31653477123/job/94302891463?pr=14399):

 FAILED tests/models/autoencoders/test_models_ltx2_diffusion_decoder.py::TestLTX2VideoDiffusionDecoderModelTiling::test_tiles_covering_the_video_match_untiled_exactly - Failed: Timeout (>60.0s) from pytest-timeout.

This PR adds a requires_accelerator decorator so that the test will be skipped on CPU to mitigate the timeout.

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