Move plugin helpers up one folder

Author: adamwathan

__tests__/plugins/util/invokePlugin.js renamed to __tests__/util/invokePlugin.js

@@ -78,7 +78,8 @@
       "<rootDir>/jest/customMatchers.js"
     ],
     "testPathIgnorePatterns": [
-      "<rootDir>/__tests__/fixtures/"
+      "<rootDir>/__tests__/fixtures/",
+      "<rootDir>/__tests__/util/"
     ]
   },
   "engines": {