refactor: move tx_template from chain to testenv

- refactor tx_template struct and init_graph
to use Cow pointer
- move `tx_template` from `chain/tests/common` to
`testenv/src`
- update `chain/tests` files to use `tx_template`
from `testenv`
- update deps in `chain/cargo.toml` and
`testenv/cargo.toml`
- update imports in `testenv/src/lib.rs`
- refactor canonical benchmarks in `chain/benches`
 to use tx_template

[Ticket: 1754]

Author: tvpeter

Diff for: crates/chain/Cargo.toml

@@ -27,7 +27,7 @@ rusqlite = { version = "0.31.0", features = ["bundled"], optional = true }
 [dev-dependencies]
 rand = "0.8"
 proptest = "1.2.0"
-bdk_testenv = { path = "../testenv", default-features = false }
+bdk_testenv = { path = "../testenv", default-features = false, features = ["miniscript"] }
 criterion = { version = "0.2" }
 
 [features]