Add ProposerSlashingBuilder

It is capable of producing double votes

eth2/types/src/proposer_slashing.rs

@@ -6,6 +6,10 @@ use serde_derive::Serialize;
 use ssz_derive::{Decode, Encode, TreeHash};
 use test_random_derive::TestRandom;
 
+mod builder;
+
+pub use builder::ProposerSlashingBuilder;
+
 #[derive(Debug, PartialEq, Clone, Serialize, Encode, Decode, TreeHash, TestRandom)]
 pub struct ProposerSlashing {
     pub proposer_index: u64,

eth2/types/src/proposer_slashing/builder.rs

@@ -0,0 +1,48 @@
+use crate::*;
+use ssz::TreeHash;
+
+pub struct ProposerSlashingBuilder();
+
+impl ProposerSlashingBuilder {
+    pub fn double_vote<F>(proposer_index: u64, signer: F, spec: &ChainSpec) -> ProposerSlashing
+    where
+        F: Fn(u64, &[u8], Epoch, u64) -> Signature,
+    {
+        let slot = Slot::new(0);
+        let shard = 0;
+
+        let proposal_data_1 = ProposalSignedData {
+            slot,
+            shard,
+            block_root: Hash256::from("one".as_bytes()),
+        };
+
+        let proposal_data_2 = ProposalSignedData {
+            slot,
+            shard,
+            block_root: Hash256::from("two".as_bytes()),
+        };
+
+        let proposal_signature_1 = {
+            let message = proposal_data_1.hash_tree_root();
+            let epoch = slot.epoch(spec.epoch_length);
+            let domain = spec.domain_proposal;
+            signer(proposer_index, &message[..], epoch, domain)
+        };
+
+        let proposal_signature_2 = {
+            let message = proposal_data_2.hash_tree_root();
+            let epoch = slot.epoch(spec.epoch_length);
+            let domain = spec.domain_proposal;
+            signer(proposer_index, &message[..], epoch, domain)
+        };
+
+        ProposerSlashing {
+            proposer_index,
+            proposal_data_1,
+            proposal_signature_1,
+            proposal_data_2,
+            proposal_signature_2,
+        }
+    }
+}