Add arbitrary(Un)RegTxCert for Imptests #16842

Job	Run time
cabal-format	7s
gen-hie	1m 25s
Check branch history	10s
build (8.10.7, ubuntu-latest)	1m 17s
fourmolu	47s
build (9.2.8, ubuntu-latest)	1m 25s
build (9.6.6, ubuntu-latest)	1m 24s
Check if formal-ledger-specifications SRP is valid	11s
build (9.8.4, ubuntu-latest)	1m 24s
build (9.10.1, ubuntu-latest)	1m 26s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
Tests completed	1s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
	9m 39s

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