tests: fix uses of `!` #28

Job	Run time
install-against-stable	7m 23s
install-against-stable	7m 23s
install-against-unstable	4m 11s
install-against-unstable	4m 11s
install-flake-against-stable	46s
install-flake-against-stable	3m 33s
install-flake-against-unstable	3m 9s
install-flake-against-unstable	3m 9s
test-stable	4m 0s
test-stable	4m 0s
test-unstable	5m 13s
test-unstable	5m 13s
	52m 11s

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