TST Run fast PEFT tests in normal CI

[BenjaminBossan]

What does this PR do?

PEFT tests were marked as slow, even though they're not slow. According to a comment there, this was probably only done to avoid running the tests before PEFT was released.

The @slow marker is now removed. All these tests use tiny models and on my machine, with warm HF cache, they passed in <12 sec on CPU without parallelism. It should thus be safe to assume that they are indeed fast enough. With these tests now running in normal CI, we should be able to prevent PEFT regressions in the future.

Note that the hotswapping tests are still marked as slow, as they partly require calling torch.compile.

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[BenjaminBossan]

@ydshieh It looks like all PEFT tests were skipped because PEFT is not installed. Should PEFT be added as a test dependency?