ci(linux): share a single Xvfb across the three test invocations

Run 27441055242 had 3-of-6 Linux jobs fail with
"xvfb-run: error: Xvfb failed to start" on the second or third
flutter test invocation. The pattern was randomly distributed across
python versions (3.12, 3.13, 3.14) and arches (AMD64, ARM64) — the
signature of a display-number race when xvfb-run is called back to
back: the previous Xvfb hadn't fully released its display before the
next xvfb-run tried to claim one.

Start one Xvfb on :99 at the top of the test step (with a kill-on-EXIT
trap), then run all three test files against the same DISPLAY.

Also bumped the post-startup settle from "none" to a fixed 1-second
sleep before the first flutter test runs (no xdpyinfo on the runner
to do a proper readiness check, and 1s is plenty for Xvfb to start
listening).

Author: FeodorFitsner

.github/workflows/ci.yml

@@ -363,12 +363,27 @@ jobs:
             --platform Linux \
             --python-version ${{ matrix.python_version }}
           set -o pipefail
+
+          # Three back-to-back `xvfb-run` invocations race for display
+          # numbers — the second/third sometimes trips "Xvfb failed to
+          # start" because the previous Xvfb's display isn't released yet
+          # (3-of-6 Linux jobs in run 27441055242 hit this). Start one
+          # Xvfb up front, share DISPLAY across all three test runs.
+          Xvfb :99 -screen 0 1280x1024x24 >/tmp/xvfb.log 2>&1 &
+          XVFB_PID=$!
+          export DISPLAY=:99
+          trap 'kill $XVFB_PID 2>/dev/null || true' EXIT
+          # Brief settle so Xvfb is accepting connections before flutter
+          # test tries to open the window. xdpyinfo isn't in the ubuntu
+          # runner's default toolchain; a fixed sleep is sufficient.
+          sleep 1
+
           # Per-file: `flutter test integration_test` (whole dir) reuses one
           # VM session and the second file fails to start the app. See
           # bridge_example_macos for the discovery.
-          xvfb-run flutter test integration_test/interactivity_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
-          xvfb-run flutter test integration_test/throughput_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
-          xvfb-run flutter test integration_test/memory_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
+          flutter test integration_test/interactivity_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
+          flutter test integration_test/throughput_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
+          flutter test integration_test/memory_test.dart -d linux --dart-define=EXPECTED_PYTHON_VERSION=${{ matrix.python_version }} -v 2>&1 | tail -300
 
       - name: Diagnostics on failure
         if: failure()