write CI which installs and executes a local electrum server on regtest

## Abstract

Currently we can execute tests locally with

`cargo test`

In addition, we can test with:

`cargo test --features nigiri`

which requires a local electrum server on regtest, called `nigiri`: https://github.com/vulpemventures/nigiri

The CI to use should be Github Actions, see an example here: https://github.com/BlockchainCommons/musign-cli/blob/master/.github/workflows/Build.yml

Both commands (`cargo test` and `cargo test --features nigiri`) should run successfully in CI