CI: Fix rake install and add Run test against installed gem

Author: junaruga

.github/workflows/ci.yml

@@ -104,7 +104,14 @@ jobs:
       # This is for ensuring using Fiddle in this repository.
       - run: ruby -Ilib test/run.rb
 
-      - run: rake install
+      - run: sudo rake install
+
+      - name: Run test against installed gem
+        run: |
+          ruby -run -e mkdir -- -p tmp/
+          ruby -run -e cp -- -pr test/ tmp/
+          cd tmp
+          ruby test/run.rb
 
   docker:
     name: >-