add --ci flag to prevent storybook opening new browser tab…

- see also storybookjs/storybook#6201

package.json

@@ -40,7 +40,7 @@
 		"test:update": "wp-scripts test-unit-js --updateSnapshot --config tests/js/jest.config.json",
 		"test:watch": "npm run test -- --watch",
 		"size-check": "bundlewatch",
-		"storybook": "start-storybook  -c ./storybook -p 6006",
+		"storybook": "start-storybook  -c ./storybook -p 6006 --ci",
 		"build-storybook": "build-storybook  -c ./storybook",
 		"changelog": "node ./bin/changelog",
 		"changelog:zenhub": "node ./bin/changelog --changelogSrcType='ZENHUB_RELEASE'"