Rename random to prev_randao (#3040)

## Issue Addressed

As discussed on last-night's consensus call, the testnets next week will target the [Kiln Spec v2](https://hackmd.io/@n0ble/kiln-spec).

Presently, we support Kiln V1. V2 is backwards compatible, except for renaming `random` to `prev_randao` in:

- https://github.com/ethereum/execution-apis/pull/180
- https://github.com/ethereum/consensus-specs/pull/2835

With this PR we'll no longer be compatible with the existing Kintsugi and Kiln testnets, however we'll be ready for the testnets next week. I raised this breaking change in the call last night, we are all keen to move forward and break things.

We now target the [`merge-kiln-v2`](https://github.com/MariusVanDerWijden/go-ethereum/tree/merge-kiln-v2) branch for interop with Geth. This required adding the `--http.aauthport` to the tester to avoid a port conflict at startup.

### Changes to exec integration tests

There's some change in the `merge-kiln-v2` version of Geth that means it can't compile on a vanilla Github runner. Bumping the `go` version on the runner solved this issue.

Whilst addressing this, I refactored the `testing/execution_integration` crate to be a *binary* rather than a *library* with tests. This means that we don't need to run the `build.rs` and build Geth whenever someone runs `make lint` or `make test-release`. This is nice for everyday users, but it's also nice for CI so that we can have a specific runner for these tests and we don't need to ensure *all* runners support everything required to build all execution clients.

## More Info

- [x] ~~EF tests are failing since the rename has broken some tests that reference the old field name. I have been told there will be new tests released in the coming days (25/02/22 or 26/02/22).~~

beacon_node/execution_layer/src/test_utils/execution_block_generator.rs

@@ -326,7 +326,7 @@ impl<T: EthSpec> ExecutionBlockGenerator<T> {
                     receipts_root: Hash256::repeat_byte(42),
                     state_root: Hash256::repeat_byte(43),
                     logs_bloom: vec![0; 256].into(),
-                    random: attributes.random,
+                    prev_randao: attributes.prev_randao,
                     block_number: parent.block_number() + 1,
                     gas_limit: GAS_LIMIT,
                     gas_used: GAS_USED,

beacon_node/execution_layer/src/test_utils/mock_execution_layer.rs

@@ -111,7 +111,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
         let parent_hash = latest_execution_block.block_hash();
         let block_number = latest_execution_block.block_number() + 1;
         let timestamp = block_number;
-        let random = Hash256::from_low_u64_be(block_number);
+        let prev_randao = Hash256::from_low_u64_be(block_number);
         let finalized_block_hash = parent_hash;
 
         self.el
@@ -120,7 +120,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
                 ExecutionBlockHash::zero(),
                 Some(PayloadAttributes {
                     timestamp,
-                    random,
+                    prev_randao,
                     suggested_fee_recipient: Address::repeat_byte(42),
                 }),
             )
@@ -133,7 +133,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
             .get_payload::<T>(
                 parent_hash,
                 timestamp,
-                random,
+                prev_randao,
                 finalized_block_hash,
                 validator_index,
             )
@@ -143,7 +143,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
         assert_eq!(payload.parent_hash, parent_hash);
         assert_eq!(payload.block_number, block_number);
         assert_eq!(payload.timestamp, timestamp);
-        assert_eq!(payload.random, random);
+        assert_eq!(payload.prev_randao, prev_randao);
 
         let status = self.el.notify_new_payload(&payload).await.unwrap();
         assert_eq!(status, PayloadStatus::Valid);