docs: document persist_test_utils module #25

Job	Run time
prepare	5s
Rust fmt	15s
Check no_std	34s
Build & Test (1.75.0, --all-features)	58s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	58s
Build & Test (1.88.0, true, --all-features)	58s
Check WASM	56s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	48s
Build & Test Examples (example_esplora)	54s
Build & Test Examples (example_electrum)	1m 0s
Build & Test Examples (example_cli)	34s
Build & Test (1.63.0, --all-features)	57s
Build & Test Examples (example_bitcoind_rpc_polling)	42s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	43s
Rust clippy	41s
	11m 3s

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