Refactor `TestEnv::wait_until_electrum_sees_block` to be more concrete. #6147

Job	Run time
prepare	4s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	1m 19s
Build and test (stable, true, --all-features)	3m 51s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	2m 0s
Check no_std	22s
Build & Test Examples (example_cli)	8s
Build and test (1.63.0, --all-features)	5m 18s
Build & Test Examples (example_bitcoind_rpc_polling)	23s
Build & Test Examples (example_electrum)	41s
Check WASM	31s
Rust fmt	13s
Build & Test Examples (example_esplora)	52s
Build & Test Examples (example_wallet_electrum)	54s
Build & Test Examples (example_wallet_esplora_async)	57s
Build & Test Examples (example_wallet_esplora_blocking)	50s
Build & Test Examples (example_wallet_rpc)	31s
clippy_check	41s
	19m 35s

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