ci: Add Hardhat tooling integration test (#516)

## Description

Adds a CI workflow that builds a small Hardhat project with
`hardhat-polkadot`, exercising the standard JSON interface between
`resolc` and `hardhat-polkadot` end-to-end.

## Resolved Issues

Closes #514

.github/workflows/test.yml

@@ -65,3 +65,7 @@ jobs:
     # Only run this if `test` successfully completes.
     needs: test
     uses: ./.github/workflows/differential-tests.yml
+
+  run-hardhat-tests:
+    needs: run-differential-tests
+    uses: ./.github/workflows/hardhat.yml