Add arbitrary(Un)RegTxCert for Imptests #16841

Job	Run time
fourmolu	49s
build (8.10.7, ubuntu-latest)	11m 3s
build (9.2.8, ubuntu-latest)	11m 3s
Check branch history	12s
build (9.6.6, ubuntu-latest)	11m 2s
cabal-format	5s
gen-hie	5m 40s
build (9.8.4, ubuntu-latest)	10m 32s
build (9.10.1, ubuntu-latest)	11m 4s
Check if formal-ledger-specifications SRP is valid	10s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
Tests completed	1s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
	1h 1m 43s

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