[CI] Add a post-install test.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -569,6 +569,16 @@ jobs:
         run: |
           ccache -s || true
 
+      - name: Install
+        run: "cmake --install /github/home/ROOT-CI/build/ --prefix /github/home/ROOT-CI/install"
+
+      - name: Post-install tests
+        run: |
+          cmake -S test/PostInstall/ -B /github/home/ROOT-CI/PostInstall -DCMAKE_PREFIX_PATH=/github/home/ROOT-CI/install -Dtesting=On;
+          cmake --build /github/home/ROOT-CI/PostInstall;
+
+      - run: ctest /github/home/ROOT-CI/PostInstall
+
   event_file:
     # For any event that is not a PR, the CI will always run. In PRs, the CI
     # can be skipped if the tag [skip-ci] or [skip ci] is written in the title.