Derive `Arbitrary` as well, fix tests #977

Job	Run time
benchcomp-tests	3m 23s
regression (macos-13)	11m 44s
perf	31m 53s
regression (ubuntu-20.04)	11m 42s
regression (ubuntu-22.04)	11m 43s
documentation	2m 1s
regression (macos-14)	11m 26s
write-json-symtab-regression	11m 44s
	1h 35m 36s

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