Add tests for legacy `Backend` impl #5094

Job	Run time
Check unused dependencies	30s
Cargo fmt	13s
Cargo check	1m 18s
Cargo check (WASM)	1m 19s
Cargo clippy	1m 5s
Check documentation and run doc tests	1m 15s
Cargo clippy (WASM)	47s
Test (WASM)	1s
Test (Unstable Backend)	1s
Test (no_std)	1s
Test (Light Client)	1s
Test (Native)	1s
	6m 32s

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