codecov doesn't cover the blockchain module

[danielabrozzoni]

Describe the bug
Codecoverage says that out coverage for the blockchain module is 1.87%, even do we do have tests covering that module. It might be configured in the wrong way.

To Reproduce
Open https://app.codecov.io/gh/bitcoindevkit/bdk, look at the end of the page, where it shows coverage for the different directories:

Expected behavior
Decent blockchain module coverage