Enable automatic tests for wxQt running on GitHub

Also build without wxUIActionSimulator support for now as it is currently
broken under wxQt and causes the tests to instantly crash if enabled.

.github/workflows/ci.yml

@@ -135,8 +135,9 @@ jobs:
           - name: Ubuntu 18.04 wxQt
             runner: ubuntu-latest
             container: ubuntu:18.04
-            configure_flags: --with-qt --enable-pch --without-opengl
+            configure_flags: --with-qt --enable-pch --without-opengl --disable-uiactionsim
             skip_samples: true
+            use_xvfb: true
 
     env:
       wxGTK_VERSION: ${{ matrix.gtk_version && matrix.gtk_version || 3 }}