Add automated test case to check org.apertis.demo.curl flatpak app

Test case allows to install and check org.apertis.demo.curl application.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

test-cases/sdk-flatpak-demo.yaml

+metadata:
+  name: sdk-flatpak-demo
+  format: "Apertis Test Definition 1.0"
+  image-types:
+    sdk:     [ amd64 ]
+  image-deployment:
+    - APT
+  type: functional
+  exec-type: automated
+  priority: medium
+  maintainer: "Apertis Project"
+  description: "Install and run org.apertis.demo.curl flatpak application."
+
+  expected:
+    - "org.apertis.demo.curl is able to retrieve https://www.apertis.org."
+
+install:
+  deps:
+    - bash
+  git-repos:
+    - url: https://gitlab.apertis.org/infrastructure/apertis-tests.git
+      branch: 'apertis/v2021dev1'
+
+run:
+  steps:
+    - "# Enter test directory:"
+    - cd apertis-tests
+    - "# Execute the following command:"
+    - common/run-test-in-systemd --user=user --basename -- sdk/automated/test-flatpak-demo.sh