refactor!: refactor tests to avoid a circular dependency with htsget-search and htsget-http

[mmalenic]

Closes #158

This PR refactors htsget-tests to avoid a circular dependency with htsget-search and htsget-http.

Implementation

• Use generic types in http-tests to avoid using the structs in htsget-search and htsget-http.
• Specify expected responses using serde_json Value. This also probably makes the tests more robust as they no longer dependent on the implementation of the htsget-http structs.
• Move CertificateKeyPair to config to avoid repeating the logic in htsget-tests and simplifying logic in htsget-search. This is a breaking change as it removes the TryFrom<DataServerConfig> for HttpTicketFormatter implementation.