Add test for the codehash of the coinbase account. (privacy-scaling-explorations#1511)

### Description
Previously we did not set the code hash for coinbase accounts in the
EndTxGadget. This was fixed in 242d38b.
This PR adds a regression test.

### Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

Co-authored-by: Mason Liang <[email protected]>

Author: z2trillion

zkevm-circuits/src/evm_circuit/execution/end_tx.rs

@@ -510,4 +510,32 @@ mod test {
             .unwrap(),
         );
     }
+
+    #[test]
+    fn end_tx_gadget_nonexisting_coinbase() {
+        // Check that the code hash of the coinbase address is correctly set to be the empty code
+        // hash when it is created because of receiving the gas fees from the first tx.
+        test_ok(
+            TestContext::<2, 2>::new(
+                None,
+                account_0_code_account_1_no_code(bytecode! {
+                    COINBASE
+                    EXTCODEHASH
+                }), /* EXTCODEHASH will return 0 for the first tx and the empty code hash for
+                     * the second tx. */
+                |mut txs, accs| {
+                    txs[0]
+                        .to(accs[0].address)
+                        .from(accs[1].address)
+                        .value(eth(1));
+                    txs[1]
+                        .to(accs[0].address)
+                        .from(accs[1].address)
+                        .value(eth(1));
+                },
+                |block, _| block,
+            )
+            .unwrap(),
+        );
+    }
 }