fix applyESLint when eslint plugin is added after unit test plugins (vuejs#5028)

* fix: fix applyESLint when eslint plugin is added after unit test plugins

closes vuejs#5017

* test: update test for mocha generator

Author: haoqunjiang

packages/@vue/cli-plugin-unit-jest/generator/index.js

@@ -18,28 +18,32 @@ module.exports = (api, _, __, invoking) => {
   })
 
   if (api.hasPlugin('eslint')) {
-    api.extendPackage({
-      eslintConfig: {
-        overrides: [
-          {
-            files: [
-              '**/__tests__/*.{j,t}s?(x)',
-              '**/tests/unit/**/*.spec.{j,t}s?(x)'
-            ],
-            env: {
-              jest: true
-            }
-          }
-        ]
-      }
-    })
+    applyESLint(api)
   }
 
   if (api.hasPlugin('typescript')) {
     applyTS(api, invoking)
   }
 }
 
+const applyESLint = module.exports.applyESLint = api => {
+  api.extendPackage({
+    eslintConfig: {
+      overrides: [
+        {
+          files: [
+            '**/__tests__/*.{j,t}s?(x)',
+            '**/tests/unit/**/*.spec.{j,t}s?(x)'
+          ],
+          env: {
+            jest: true
+          }
+        }
+      ]
+    }
+  })
+}
+
 const applyTS = (module.exports.applyTS = (api, invoking) => {
   api.extendPackage({
     jest: {

packages/@vue/cli-plugin-unit-mocha/__tests__/mochaGenerator.spec.js

@@ -17,7 +17,6 @@ test('base', async () => {
 
   expect(pkg.scripts['test:unit']).toBe('vue-cli-service test:unit')
   expect(pkg.devDependencies).toHaveProperty('@vue/test-utils')
-  expect(files['tests/unit/.eslintrc.js']).toMatch('mocha: true')
 
   const spec = files['tests/unit/example.spec.js']
   expect(spec).toMatch(`import { expect } from 'chai'`)

packages/@vue/cli-plugin-unit-mocha/generator/index.js

@@ -15,18 +15,6 @@ module.exports = (api, _, __, invoking) => {
 
   if (api.hasPlugin('eslint')) {
     applyESLint(api)
-    api.extendPackage({
-      eslintConfig: {
-        overrides: [
-          {
-            files: ['**/__tests__/*.{j,t}s?(x)'],
-            env: {
-              mocha: true
-            }
-          }
-        ]
-      }
-    })
   }
 
   if (api.hasPlugin('typescript')) {
@@ -35,10 +23,20 @@ module.exports = (api, _, __, invoking) => {
 }
 
 const applyESLint = module.exports.applyESLint = api => {
-  api.render(files => {
-    files['tests/unit/.eslintrc.js'] = api.genJSConfig({
-      env: { mocha: true }
-    })
+  api.extendPackage({
+    eslintConfig: {
+      overrides: [
+        {
+          files: [
+            '**/__tests__/*.{j,t}s?(x)',
+            '**/tests/unit/**/*.spec.{j,t}s?(x)'
+          ],
+          env: {
+            mocha: true
+          }
+        }
+      ]
+    }
   })
 }