add test script

flutter/flutter-test.gitlab-ci.yml

+stages:
+- test
+
+variables:
+  FLUTTER_TEST_IMAGE: andrijoos/flutter:3-linux
+  APP_DIRECTORY: "."
+
+.run_tests:
+  stage: test
+  image: ${FLUTTER_TEST_IMAGE}
+  tags:
+  - amd64
+  script:
+  - cd ${APP_DIRECTORY}
+  - flutter test ${TEST_ARGS}