CI Linux: run tests under the offscreen QPA

test_aopacket uses QTEST_MAIN, which builds a QGuiApplication and needs
a platform plugin. The Linux runner is headless — xcb can't connect to
a display and the test aborts before its first assertion. Force the
offscreen QPA for the ctest step so QGuiApplication initialises against
a memory-backed surface. test_loopsidecar (QTEST_APPLESS_MAIN) was
unaffected. Windows runners have a real display so no equivalent fix
is needed there.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: OmniTroid

.github/workflows/build.yml

@@ -89,6 +89,8 @@ jobs:
         run: ./configure.sh QT_PATH="$QT_ROOT_DIR" BUILD_TYPE=Release
 
       - name: Run tests
+        env:
+          QT_QPA_PLATFORM: offscreen
         run: ctest --output-on-failure
 
       - name: Stage APNG plugin for AppImage