Use posix paths for Jest config during eject (#1635)

Resolves #1417 and #1498.

packages/react-scripts/scripts/eject.js

@@ -129,7 +129,7 @@ prompt(
   // Add Jest config
   console.log('  Adding ' + cyan('Jest') + ' configuration');
   appPackage.jest = createJestConfig(
-    filePath => path.join('<rootDir>', filePath),
+    filePath => path.posix.join('<rootDir>', filePath),
     null,
     true
   );

packages/react-scripts/scripts/test.js

@@ -32,7 +32,7 @@ const createJestConfig = require('../utils/createJestConfig');
 const path = require('path');
 const paths = require('../config/paths');
 argv.push('--config', JSON.stringify(createJestConfig(
-  relativePath => path.resolve(__dirname, '..', relativePath),
+  relativePath => path.posix.resolve(__dirname.replace(/[\\]+/g, path.posix.sep), '..', relativePath),
   path.resolve(paths.appSrc, '..'),
   false
 )));