[Improvement] Test: separate jest config from packages.json (#1218)

jest.config.js

@@ -0,0 +1,23 @@
+module.exports = {
+  moduleFileExtensions: ['js', 'vue'],
+  transform: {
+    '^.+\\.js$': '<rootDir>/node_modules/babel-jest',
+    '.*\\.(vue)$': '<rootDir>/node_modules/vue-jest'
+  },
+  moduleNameMapper: {
+    '^packages/(.*)$': '<rootDir>/packages/$1'
+  },
+  snapshotSerializers: ['<rootDir>/node_modules/jest-serializer-vue'],
+  collectCoverageFrom: [
+    'packages/**/*.{js,vue}',
+    '!**/vant-css/**',
+    '!**/demo/**',
+    '!**/locale/lang/**',
+    '!**/waterfall/**',
+    '!**/sku/**',
+    '!**/lazyload/**'
+  ],
+  collectCoverage: true,
+  coverageReporters: ['html', 'lcov', 'text-summary'],
+  coverageDirectory: './test/coverage'
+};