FFT / DeEmbed tests: use fftwf_destroy_plan not fftwf_free to dealloc… #585

Job	Run time
Linux (debian-stable, trixie, debian:trixie, repo, true)	3m 34s
Linux (debian-oldstable, bookworm, debian:bookworm, repo, false)	2m 22s
Linux (debian-oldstable, bookworm, debian:bookworm, repo, true)	3m 25s
Linux (debian-oldstable, bookworm, debian:bookworm, standalone, false)	3m 32s
Linux (debian-stable, trixie, debian:trixie, repo, false)	2m 25s
	15m 18s

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