Add option to test phase to generate junit output

[x] I understand that GitHub issues are not for tech support, but for questions specific to this generator, bug reports, and feature requests.

Item	Version
generator-angular-fullstack	4.0.0
Node	x.x.x
npm	x.x.x
Operating System	OS X 10 / Windows 10 / Ubuntu 15.10 / etc
etc	etc

Item	Answer
Transpiler	Babel / TypeScript
Markup	HTML / Jade
CSS	CSS / LESS / SCSS / Stylus
Router	ngRoute / ui-router
Build Tool	Gulp
Client Tests	Mocha
DB	MongoDB / SQL
Auth	Y / N
etc	etc

It would be good to have a feature where the test phase can generate optional junit XML files e.g. gulp test:junit This would make the tests for the generator itself more visible to circleci https://circleci.com/docs/test-metadata/ which I think is the CI for v4.0.0 This would also help users of the generator as junit is a standard in Jenkins, Bamboo and TeamCity

I think the tricky part will be assembling the tests from all the temporary apps which are part of the testing

I looked into this as I need the tests to pass so circleci can go ahead and create a branch for the docker image https://github.com/angular-fullstack/generator-angular-fullstack/issues/1959

such as in circle.yml

## Deployments
deployment:
  docker:
    # update the upstream dockerfile after a push
    branch: [master, docker-integration]
    commands:
      - generator-angular-fullstack/angular-fullstack-dockerfile/create_branch.sh

angular-fullstack / generator-angular-fullstack

Add option to test phase to generate junit output #2066