ci: add github action to test the snap

Add github action to test that the snap package was generated
correctly, this CI don't test the snap, it just build it.

fixes #838

Signed-off-by: Julio Montes <julio.montes@intel.com>

.github/workflows/snap.yaml

@@ -0,0 +1,15 @@
+name: snap CI
+on: ["pull_request"]
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Check out
+        uses: actions/checkout@v2
+
+      - name: Install Snapcraft
+        uses: samuelmeuli/action-snapcraft@v1
+
+      - name: Build snap
+        run: |
+          snapcraft -d snap --destructive-mode