start an embedded ES node for tests

This should mean the native tests can run on CI again.
Currently the embedded node is always started, and port is hardcoded to avoid conflicting with a running external node. 
I've left the connection going through the existing connect code, but it would be possible to call (. node connect) to get a local connection instead.
This change also means that the tests will always run against the version of ES specified in the project.clj  - the native clients aren't compatible across major version changes, so this makes sense, but should probably be documented somewhere.