Ensure VALID response from fcU updates protoarray (#3126)

## Issue Addressed

NA

## Proposed Changes

Ensures that a `VALID` response from a `forkchoiceUpdate` call will update that block in `ProtoArray`.

I also had to modify the mock execution engine so it wouldn't return valid when all payloads were supposed to be some other static value.

## Additional Info

NA

consensus/proto_array/src/proto_array_fork_choice.rs

@@ -188,6 +188,16 @@ impl ProtoArrayForkChoice {
         })
     }
 
+    /// See `ProtoArray::propagate_execution_payload_validation` for documentation.
+    pub fn process_execution_payload_validation(
+        &mut self,
+        block_root: Hash256,
+    ) -> Result<(), String> {
+        self.proto_array
+            .propagate_execution_payload_validation(block_root)
+            .map_err(|e| format!("Failed to process valid payload: {:?}", e))
+    }
+
     /// See `ProtoArray::propagate_execution_payload_invalidation` for documentation.
     pub fn process_execution_payload_invalidation(
         &mut self,