Move ITs into their own crate #599

Job	Run time
Check formatting	15s
Check clippy	37s
Run unit tests (ubuntu-latest)	53s
Run unit tests (macOS-latest)	1m 8s
Validate minimal dependency versions	14s
Run integration tests (5)	25s
Run integration tests (2025)	24s
Run unit tests (windows-latest)	36s
Run integration tests (4.4)	18s
Release	0s
	4m 50s

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