Use non-zero costmodels in Imp tests #16454

Job	Run time
fourmolu	44s
cabal-format	6s
build (8.10.7, ubuntu-latest)	22m 58s
build (9.2.8, ubuntu-latest)	13m 54s
Check branch history	11s
build (9.6.6, ubuntu-latest)	16m 33s
build (9.8.2, ubuntu-latest)	11m 26s
gen-hie	5m 24s
build (9.10.1, ubuntu-latest)	13m 33s
Check if formal-ledger-specifications SRP is valid	18s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	0s
Tests completed	1s
Send a slack notification on \#ledger-internal if the nightly build failed	0s
	1h 25m 8s

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