fix(generator/mocha): non webpack require spectron

Author: nklayman

generator/templates/tests-mocha/tests/unit/electron.spec.js

@@ -1,4 +1,5 @@
-import spectron from 'spectron'
+// eslint-disable-next-line no-undef
+const spectron = __non_webpack_require__('spectron')
 import testWithSpectron from 'vue-cli-plugin-electron-builder/lib/testWithSpectron'
 import chai from 'chai'
 import chaiAsPromised from 'chai-as-promised'