add reconnecting tests for unstable_backend #5192

Job	Run time
Cargo fmt	11s
Check unused dependencies	32s
Cargo check (WASM)	1m 45s
Cargo clippy (WASM)	45s
Cargo clippy	6m 21s
Cargo check	7m 19s
Check documentation and run doc tests	6m 44s
Test (no_std)	2m 14s
Test (Light Client)	3m 6s
Test (Unstable Backend)	12m 37s
Test (WASM)	11m 32s
Test (Native)	12m 48s
	1h 5m 54s

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