Fix some hive tests

[somnathb1]

Changes:
Reorder some Engine API invalid checks to be closer to specs/hive tests
Move the Engine API direct method names to interfaces

[yperbasis]

Why move some methods to a different file? The old way when all EngineServer methods live in engine_server.go looks logical to me. Could you put them back please.

N.B. Ideally semantic changes should be decoupled from cosmetic reshuffling (though sometimes it's impractical to do that). Otherwise it's difficult to review.

[somnathb1]

Why move some methods to a different file? The old way when all EngineServer methods live in engine_server.go looks logical to me. Could you put them back please.

N.B. Ideally semantic changes should be decoupled from cosmetic reshuffling (though sometimes it's impractical to do that). Otherwise it's difficult to review.

Well, the logic part of the implementation of the interface methods actually lie within engine_server.go. The old way here, makes things a tad more difficult to make changes to.
The methods now moved to interfaces.go would more likely not be changed at all, so it makes sense to separate them.
However, I have put the methods back. @yperbasis