github actions: Report failure on warnings

Make it more clear in the status check if there are audit warnings,
which includes yanked and unmaintained crates.

Also check the test client.

.github/workflows/audit.yml

@@ -19,5 +19,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
 
-      - name: Cargo audit
-        run: cargo audit
+      - name: Audit server
+        run: cargo audit --deny warnings
+
+      - name: Audit test client
+        run: cargo audit --deny warnings
+        working-directory: misc/test-client