fix(cli-plugin-unit-jest): When using TS & Babel, make ts-jest use babelConfig (#3107)

* fix(cli-plugin-unit-jest): use ts-jest babelConfig

When using Typescript and Babel, ts-jest should use babel to transform result for correct JSX transforms

close #3100

* refactor:  write generator a bit differently.

previous way could end up with ugly `global: undefined`
when not using babel

Author: LinusBorg

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

@@ -88,6 +88,14 @@ const applyTS = module.exports.applyTS = (api, invoking) => {
   })
   if (api.hasPlugin('babel')) {
     api.extendPackage({
+      jest: {
+        global: {
+          'ts-jest': {
+            // we need babel to transpile JSX
+            babelConfig: true
+          }
+        }
+      },
       devDependencies: {
         // this is for now necessary to force ts-jest and vue-jest to use babel 7
         'babel-core': '7.0.0-bridge.0'