Add functions to test persistence

[110CodingP]

Description

Added basic tests to the testenv for testing custom persistence backends. Partially solves bdk_wallet#14 and might help with bdk_wallet#234.

Notes to the reviewers

An alternative approach could be to create a struct whose methods are these tests but the current approach seems better since these tests are meant to be used as unit tests. For the same reason redundant functions to check persistence of individual fields of the ChangeSets are provided.

Changelog notice

Added
   - `miniscript` feature to testenv which depends on `bdk_chain/miniscript` .
    - functions to testenv to check `ChangeSet`s are persisted and merged `ChangeSet`s are loaded properly ,gated by the `miniscript` and `std` feature flags.
    - tests to `file_store` and `chain` based on utilities added to `testenv`.
 Changed
   - default feature of testenv to also activate `miniscript` feature.

Todo

• Add docs
• Add tests to bdk_chain based on testenv utilities

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature