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

[tvpeter]

Description

This PR implements the following changes:

• moves TxTemplate from crates/chain/tests/common/mod.rs to testenv.
• refactors init_graph function and TxTemplate struct to use Cow
• redoes the canonical benchmark tests introduced by bitcoindevkit/bdk#1670 using TxTemplates

Closes bitcoindevkit/bdk#1936

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

[oleonardolima]

You probably need a rebase to make sure CI is passing.