tests: fix test_fts_distributed_compat on MacOS

[booxter]

What does this PR do?

Fix test_fts_distributed_compat on MacOS.

lightning raises the following error when initializing the Trainer:

ValueError: You set `strategy=ddp` but strategies from the DDP family
are not supported on the MPS accelerator. Either explicitly set
`accelerator='cpu'` or change the strategy.

Does your PR introduce any breaking changes? If yes, please list them.

None.

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[speediedan]

Thanks for the good catch and appreciated contribution @booxter! Best of luck with your work and let me know if you have any further suggestions.