TestServer does not allow KolasuServer subclasses to be used

[loradd]

The current implementation of the TestServer does not allow users to customize the underlying server to use - KolasuServer is used by default. It must be possible to configure this for those cases where a KolasuServer subclass is required.

Enhancement Proposal
After a discussion with @martin-azpillaga, we propose to expose a server: KolasuServer argument in the TestServer constructor.

[loradd]

This might not be required given #69 @martin-azpillaga