Add integration test for fleetctl preview (#3545)

Add nightly and manual job to run `fleetctl preview` and check for enrolled hosts.

Works on macOS and Linux. Windows cannot be tested on GitHub Actions due to lack of Docker Linux support.

.github/workflows/fleetctl-preview.yml

@@ -0,0 +1,42 @@
+name: Test fleetctl preview
+
+on:
+  workflow_dispatch: # Manual
+  schedule:
+  - cron: '0 2 * * *' # Nightly 2AM UTC
+
+jobs:
+  test-preview:
+    timeout-minutes: 60
+    strategy:
+      matrix:
+        # Doesn't work on Windows because Linux Docker containers are not supported.
+        os: [ubuntu-20.04, ubuntu-18.04, macos-10.15, macos-11]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Install Docker
+      # Docker needs to be installed manually on macOS.
+      if: contains(matrix.os, 'macos')
+      # From https://github.com/docker/for-mac/issues/2359#issuecomment-943131345
+      run: |
+        brew install --cask docker
+        sudo /Applications/Docker.app/Contents/MacOS/Docker --unattended --install-privileged-components
+        open -a /Applications/Docker.app --args --unattended --accept-license
+        echo "Waiting for Docker to start up..."
+        while ! /Applications/Docker.app/Contents/Resources/bin/docker info &>/dev/null; do sleep 1; done
+        echo "Docker is ready."
+
+    - name: Start tunnel
+      run: |
+        npm install -g localtunnel
+        lt --port 1337 &
+        sleep 5
+
+    - name: Test fleetctl preview
+      run: |
+        npm install -g fleetctl
+        fleetctl preview
+        sleep 10
+        fleetctl get hosts | tee hosts.txt
+        [ $( cat hosts.txt | grep online | wc -l) -eq 8 ]
+      shell: bash