fix gate-ing of atomic64 in tests #1284

Job	Run time
check-docs	46s
external-types	32s
cargo-hack	1m 7s
check	44s
cargo-deny check (bans licenses sources)	21s
cargo-deny check (advisories)	21s
test-msrv	3m 54s
style	9s
test-versions (beta)	1m 13s
test-versions (stable)	1m 14s
test-versions (nightly)	1m 36s
	11m 57s

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