Refactor `TestEnv::wait_until_electrum_sees_block` to be more concrete. #6148

Job	Run time
prepare	2s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	1m 26s
Check no_std	14s
Check WASM	59s
Build and test (stable, true, --all-features)	3m 52s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	1m 58s
Build and test (1.63.0, --all-features)	4m 52s
Build & Test Examples (example_cli)	12s
Rust fmt	5s
Build & Test Examples (example_bitcoind_rpc_polling)	29s
Build & Test Examples (example_electrum)	45s
Build & Test Examples (example_esplora)	47s
Build & Test Examples (example_wallet_electrum)	47s
Build & Test Examples (example_wallet_esplora_async)	56s
Build & Test Examples (example_wallet_esplora_blocking)	53s
Build & Test Examples (example_wallet_rpc)	27s
clippy_check	48s
	19m 32s

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