Ignore the yarn cache directory (#2063)

Author: jmorrell

packages/react-scripts/scripts/utils/createJestConfig.js

@@ -26,7 +26,18 @@ module.exports = (resolve, rootDir, isEjecting) => {
     setupFiles: [resolve('config/polyfills.js')],
     setupTestFrameworkScriptFile: setupTestsFile,
     testPathIgnorePatterns: [
-      '<rootDir>[/\\\\](build|docs|node_modules|scripts)[/\\\\]',
+      // Ignore the following directories:
+      // build
+      //   - the build output directory
+      // .cache
+      //   - the yarn module cache on Ubuntu if $HOME === rootDir
+      // docs
+      //   - often used to publish to Github Pages
+      // node_modules
+      //   - ignore tests in dependencies
+      // scripts
+      //   - directory generated upon eject
+      '<rootDir>[/\\\\](build|\\.cache|docs|node_modules|scripts)[/\\\\]',
     ],
     testEnvironment: 'node',
     testURL: 'http://localhost',