chore: extract `TestEnv` into separate crate #4239

Job	Run time
clippy_check	43s
Check WASM	58s
Build and test (stable, true, --no-default-features)	3m 22s
Check no_std	38s
Build and test (stable, true, --all-features)	3m 51s
Rust fmt	8s
Build and test (1.57.0, --no-default-features)	2m 49s
Build and test (1.57.0, --all-features)	4m 26s
	16m 55s

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