simulators/ethereum/engine: Refactor Test Verification Syntax

[marioevz]

This PR includes a refactoring of the Test Verification syntax to make the test cases much more readable and explicit on what they expect as outcome.

The Expect* methods are introduced to allow test cases to explicitly indicate what to expect from each call they perform, to either the Engine API or the Eth JSON-RPC.

E.g. this verification:

fcResp, err := t.Engine.EngineForkchoiceUpdatedV1(t.Engine.Ctx(), &forkchoiceState, nil)
if err != nil {
	t.Fatalf("FAIL (%s): ForkchoiceUpdated under PoW rule returned error (Expected INVALID_TERMINAL_BLOCK): %v, %v", t.TestName, err)
}
if fcResp.PayloadStatus.Status != "INVALID_TERMINAL_BLOCK" {
	t.Fatalf("INFO (%v): Incorrect EngineForkchoiceUpdatedV1 response for invalid PoW parent (Expected PayloadStatus.Status=INVALID_TERMINAL_BLOCK): %v", t.TestName, fcResp.PayloadStatus.Status)
}
if fcResp.PayloadStatus.LatestValidHash != nil {
	t.Fatalf("INFO (%v): Incorrect EngineForkchoiceUpdatedV1 response for invalid PoW parent (Expected PayloadStatus.LatestValidHash==nil): %v", t.TestName, fcResp.PayloadStatus)
}

becomes:

t.TestEngine.
	TestEngineForkchoiceUpdatedV1(&forkchoiceState, nil).
	ExpectPayloadStatus(InvalidTerminalBlock).
	ExpectLatestValidHash(nil)

The verification fail check is now embedded within all the Expect* methods and, unless they explicitly expect an error (e.g. ExpectError), the call automatically checks for err == nil.

[fjl]

The method chaining pattern is cute, but not typically used in Go code. It is preferable to assign the receiver to a single letter variable and then call the methods on that, e.g.

r := t.TestEngine.TestEngineForkchoiceUpdatedV1(&forkchoiceState, nil)
r.ExpectPayloadStatus(InvalidTerminalBlock)
r.ExpectLatestValidHash(nil)

The reasoning behind this is:

• There are usually none (or very few) LoC saved by chaining.
• It can be more difficult to insert/remove checks if chaining is used. For example, when removing the last chained call, the . on the previous line must be removed also.

[fjl]

Note, this means Expect* should have no return value.

[fjl]

Thanks, it looks a bit better with your changes.