Add tests for legacy `Backend` impl #5092

Job	Run time
Cargo fmt	16s
Check unused dependencies	26s
Cargo check	8m 57s
Cargo clippy	8m 45s
Check documentation and run doc tests	8m 45s
Cargo check (WASM)	1m 41s
Cargo clippy (WASM)	53s
Test (no_std)	1s
Test (Unstable Backend)	1s
Test (WASM)	1s
Test (Native)	1s
Test (Light Client)	1s
	29m 48s

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