docs: document persist_test_utils module #28

Job	Run time
prepare	4s
Rust fmt	17s
Build & Test (1.63.0, --all-features)	5m 34s
Check WASM	1m 15s
Rust clippy	1m 23s
Build & Test (1.88.0, true, --all-features)	3m 41s
Build & Test Examples (example_cli)	35s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	4m 19s
Check no_std	41s
Build & Test Examples (example_esplora)	55s
Build & Test (1.75.0, --all-features)	3m 55s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 9s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	1m 54s
Build & Test Examples (example_bitcoind_rpc_polling)	47s
Build & Test Examples (example_electrum)	59s
	28m 28s

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