Use Xvfb for Linux desktop tests (flutter#2750)

Flutter plugin tests don't necessarily run in headless mode; this adds a virtual display when running Linux tests so that they will run in normal mode.

Fixes flutter/flutter#55327

.ci/Dockerfile-LinuxDesktop

@@ -15,5 +15,7 @@ RUN sudo apt-get update && sudo apt-get install -y google-cloud-sdk && \
     gcloud config set core/disable_usage_reporting true && \
     gcloud config set component_manager/disable_update_check true
 
+# Install xvfb to allow running headless
+RUN sudo apt-get install -y xvfb libegl1-mesa
 # Install Linux desktop requirements.
 RUN sudo apt-get install -y clang make cmake ninja-build rsync pkg-config

.cirrus.yml

@@ -121,7 +121,7 @@ task:
         # TODO(stuartmorgan): Include stable once Linux is supported on stable.
         - flutter channel master
         - ./script/incremental_build.sh build-examples --linux
-        - ./script/incremental_build.sh drive-examples --linux
+        - xvfb-run ./script/incremental_build.sh drive-examples --linux
 
 task:
   # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins