run only one test at a time again

package.json

@@ -35,7 +35,7 @@
     "remove:dependency": "node buildutils/lib/remove-dependency.js",
     "remove:package": "node buildutils/lib/remove-package.js",
     "remove:sibling": "node buildutils/lib/remove-package.js",
-    "test": "lerna run test:chrome-headless --scope \"@jupyterlab/test-*\" --stream",
+    "test": "lerna run test:chrome-headless --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",
     "test:chrome": "lerna run test:chrome --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",
     "test:chrome-headless": "lerna run test:headless --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",
     "test:firefox": "lerna run test:firefox --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",