Revert Jest moduleNameMapper regex back to a whitelist (#1149)

Jest matches moduleNameMapper regexes with module names, not the full
file path, so the negative lookahead doesn’t work for filtering out
JS files, because they can be imported without the extension. So paths
like `lodash.assign` and `../utils/range` were mislabeled as resources
with unknown file extensions because they have a dot in the name.

As a stopgap measure, revert the moduleNameMapper regex added in #1077.

packages/react-scripts/utils/createJestConfig.js

@@ -20,7 +20,7 @@ module.exports = (resolve, rootDir, isEjecting) => {
   const config = {
     collectCoverageFrom: ['src/**/*.{js,jsx}'],
     moduleNameMapper: {
-      '^.+\\.(?!(js|jsx|css|json)$)[^\\.]+$': resolve('config/jest/FileStub.js'),
+      '^.+\\.(ico|jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$': resolve('config/jest/FileStub.js'),
       '^.+\\.css$': resolve('config/jest/CSSStub.js')
     },
     setupFiles: [resolve('config/polyfills.js')],