docs: document persist_test_utils module #27

Job	Run time
Rust fmt	14s
prepare	6s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	1m 53s
Rust clippy	1m 21s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 44s
Build & Test Examples (example_cli)	38s
Check no_std	37s
Build & Test Examples (example_bitcoind_rpc_polling)	44s
Build & Test (1.63.0, --all-features)	2m 44s
Check WASM	1m 0s
Build & Test (1.75.0, --all-features)	2m 44s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 17s
Build & Test (1.88.0, true, --all-features)	2m 33s
Build & Test Examples (example_electrum)	59s
Build & Test Examples (example_esplora)	55s
	21m 29s

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