Migrate test_grakn and test_debug to BDD

[alexjpwalker]

Problem to Solve

test_grakn.py and test_debug.py contain some useful test scenarios that should be migrated to BDD. For example: explanation tests, and Transaction Options tests.

Proposed Solution

We should:

1. pick out the useful scenarios
2. migrate them to BDD scenarios
3. implement these scenarios in Grakn Core and all clients
4. delete test_grakn, but retain test_debug as it's useful for debugging issues locally.