Upgrade @web/test-runner. (rhashimoto#257)

Co-authored-by: Roy Hashimoto <roy@shoestringresearch.com>

Author: rhashimoto

package.json

@@ -24,8 +24,8 @@
   "devDependencies": {
     "@types/jasmine": "^5.1.4",
     "@web/dev-server": "^0.4.6",
-    "@web/test-runner": "^0.18.2",
-    "@web/test-runner-core": "^0.13.3",
+    "@web/test-runner": "^0.20.0",
+    "@web/test-runner-core": "^0.13.4",
     "comlink": "^4.4.1",
     "jasmine-core": "^4.5.0",
     "monaco-editor": "^0.34.1",