Improve structure of test targets

[alexjpwalker]

Problem to Solve

In client-java, we have separated out the Core and Cluster tests. I'm not sure if this is really a good idea, we should review it. Also, it has resulted in a huge number of CI jobs, which feels excessive.

Whatever we do, it should be propagated to the other clients. The other clients currently have no separation between Core and Cluster.

Proposed Solution

We could either:

• bring the Core and Cluster tests together again, or
• bring the various Graql tests together again (definable, writable, readable)

Could even do both.

[alexjpwalker]

This change was propagated to all clients.