test: Add `leaf_version` parameter to `taproot_tree_helper()`

This PR adds a leaf_version parameter to taproot_tree_helper(). Previously the leaf version was hard coded, because we only currently support 1 leaf version.

Proposed soft forks such as #29221 require new leaf versions to be allocated. This PR allows the test framework to accomodate new leaf versions. This commit is also included in #29221, but in the policy of trying to avoid mega-PRs, I carved this out into a separate PR that can be merged.

This PR does not alter any test functionality, just adds a parameter and uses it across the codebase rather than hard coding the value inside of taproot_tree_helper().