ci: run rpctest integration tests

`make build`, `make unit-cover`, and `make unit-race` all use plain
`go test` without the rpctest build tag, so anything under
//go:build rpctest -- the entire integration/ package outside of
rpctest/, and parts of rpctest/ itself -- is not exercised by CI.
Bugs that only surface under -tags=rpctest can land on master without
detection.

Add a test-rpctest job that runs `make unit` (which sets
-tags=rpctest) so rpctest-tagged tests are part of every push and PR.

.github/workflows/main.yml

@@ -145,3 +145,18 @@ jobs:
 
       - name: Test
         run: make unit-race
+
+  test-rpctest:
+    name: Unit rpctest
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ env.GO_VERSION }}
+
+      - name: Check out source
+        uses: actions/checkout@v4
+
+      - name: Test
+        run: make unit