github: run fuzz tests with unit tests

We download and extract the fuzzing seed corpora into the LND tree so
that "make unit" automatically runs the fuzz tests on the seeds.

.github/workflows/main.yml

@@ -167,6 +167,15 @@ jobs:
       - name: git checkout
         uses: actions/checkout@v3
 
+      - name: git checkout fuzzing seeds
+        uses: actions/checkout@v3
+        with:
+          repository: lightninglabs/lnd-fuzz
+          path: lnd-fuzz
+
+      - name: rsync fuzzing seeds
+        run: rsync -a --ignore-existing lnd-fuzz/ ./
+
       - name: setup go ${{ env.GO_VERSION }}
         uses: ./.github/actions/setup-go
         with: