[FIX] align F tests specification with BIDS stats model #983

Job	Run time
tests (system, moae, ubuntu-latest, R2023b)	26m 6s
tests (system, moae, macos-13, R2023b)	40m 10s
tests (system, facerep, ubuntu-latest, R2023b)	36m 34s
tests (system, facerep, macos-13, R2023b)	59m 9s
tests (system, fmriprep, ubuntu-latest, R2023b)	10m 21s
tests (system, fmriprep, macos-13, R2023b)	9m 52s
tests (system, ubuntu-latest, R2023b, bms)	18m 19s
tests (unit, ubuntu-latest, R2023b, fast)	7m 33s
tests (unit, macos-13, R2023b, fast)	13m 41s
tests (unit, ubuntu-latest, R2023b, slow)	13m 18s
tests (unit, macos-13, R2023b, slow)	25m 49s
tests (unit, macos-13, R2023b, fast)	16m 35s
	4h 37m 27s

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