Merge bitcoindevkit/bdk#545: Update `electrsd` to v0.15

1803f5ea8a7a2f8f9ebdfa520baad91f84bd6ba0 Update `electrsd` to v0.15 (Alekos Filini)

Pull request description:

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  ### Description

  PR RCasatta/electrsd#33 has been merged and the new changes released as part of `v0.15.0`: the PR allows setting an env variable (`ELECTRSD_EXE`) to use a custom electrsd binary, which can be useful when the one that is downloaded automatically doesn't work for whatever reason.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [ ] I've updated `CHANGELOG.md`

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  LLFourn:
    ACK 1803f5e
  notmandatory:
    ACK 1803f5ea8a7a2f8f9ebdfa520baad91f84bd6ba0

Tree-SHA512: 01d84763a5acf3aa1686b91ba7cb0c7ec3a0ca6c7b9bac29aa74a72f54ed31c476b7a8b0f8c6c8a8ccf530ad8a19e90e144ac4cacbe436699ef9e319b5732f1c

diff --cc src/testutils/blockchain_tests.rs
Simple merge