Add comments to build_and_test_linux_unopt_debug (flutter/engine#17567)

Clarify that the test depends on the framework repo. It may fail if the
framework repo is broken, so the engine committer don't need to be in
panic.

engine/src/flutter/.cirrus.yml

@@ -74,6 +74,9 @@ task:
       test_host_script: |
         cd $ENGINE_PATH/src
         ./flutter/testing/run_tests.sh host_release
+
+    # The following test depends on Flutter framework repo. It may fail if the
+    # framework repo is currently broken.
     - name: build_and_test_linux_unopt_debug
       compile_host_script: |
         cd $ENGINE_PATH/src
@@ -97,6 +100,9 @@ task:
         mkdir -p $FRAMEWORK_PATH
         cd $FRAMEWORK_PATH
         git clone https://github.com/flutter/flutter.git
+      verify_framework_script:
+        - echo "Checking that the framework Cirrus test is not currently broken..."
+        - curl -s https://api.cirrus-ci.com/github/flutter/flutter.json | grep -q passing
       test_web_script: |
         cd $FRAMEWORK_PATH/flutter/dev/integration_tests/web
         ../../../bin/flutter config --local-engine=host_debug_unopt --no-analytics --enable-web