chore: extract `TestEnv` into separate crate #4211

Job	Run time
Check no_std	1m 7s
Build and test (stable, true, --no-default-features)	3m 24s
Build and test (stable, true, --all-features)	3m 20s
Build and test (1.57.0, --no-default-features)	3m 12s
Check WASM	1m 7s
Build and test (1.57.0, --all-features)	3m 28s
Rust fmt	16s
clippy_check	4m 46s
	20m 40s

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