refactor: move `tx_template` from `bdk_chain` to `bdk_testenv` #6694

Job	Run time
prepare	4s
Check no_std	26s
Check WASM	1m 3s
Build & Test Examples (example_cli)	29s
Build & Test (1.84.0, true, --no-default-features --features miniscript/no-std)	2m 21s
Build & Test Examples (example_bitcoind_rpc_polling)	34s
Rust clippy	47s
Rust fmt	11s
Build & Test (1.84.0, true, --all-features)	3m 53s
Build & Test Examples (example_electrum)	1m 2s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std)	3m 16s
Build & Test Examples (example_esplora)	1m 2s
Build & Test (1.63.0, --all-features)	4m 33s
Build & Test Examples (example_wallet_electrum)	1m 0s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std)	2m 32s
Build & Test Examples (example_wallet_esplora_async)	1m 2s
Build & Test (1.75.0, --all-features)	4m 6s
Build & Test Examples (example_wallet_esplora_blocking)	1m 2s
Build & Test Examples (example_wallet_rpc)	40s
	30m 3s

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