chore: extract `TestEnv` into separate crate #4264

Job	Run time
Build and test (stable, true, --no-default-features)	2m 0s
Check no_std	35s
Build and test (stable, true, --all-features)	2m 11s
Build and test (1.57.0, --no-default-features)	5m 43s
Check WASM	46s
clippy_check	38s
Build and test (1.57.0, --all-features)	2m 49s
Rust fmt	6s
	14m 48s

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