Exclude storybook (*.stories) from coverage collection (#22661)

We are not running tests which exercise those *.stories files, hence it
is not fair to include them in the coverage analysis.

jest.unit.conf.json

@@ -26,7 +26,8 @@
   "coverageReporters": ["text", "html", "lcov"],
   "collectCoverageFrom": [
     "frontend/src/**/*.{js,jsx,ts,tsx}",
-    "enterprise/frontend/src/**/*.{js,jsx,ts,tsx}"
+    "enterprise/frontend/src/**/*.{js,jsx,ts,tsx}",
+    "**/*.stories.{js,jsx,ts,tsx}"
   ],
   "coveragePathIgnorePatterns": [
     "/node_modules/",