Use screen option to try fix ci

As suggested here:
microsoft/vscode#95415 (comment)

Author: jneira

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
           node-version: 10.x
       - run: npm ci
       - run: npm run webpack
-      - run: xvfb-run -a npm test
+      - run: xvfb-run -s '-screen 0 1024x768x24' -a npm test
         if: runner.os == 'Linux'
       - run: npm test
         if: runner.os != 'Linux'