Stimulus: Add testing in the docs

[guledali]

https://stimulusjs.org/handbook/ Please add information regarding testing in the handbook.

[guledali]

Im going to close this one, they mention in a different issue https://github.com/stimulusjs/stimulus/issues/34 That testing is done with Capybara and selenium here is a link https://guides.rubyonrails.org/testing.html#system-testing

If you are not using rails then https://www.cypress.io/ might be a good option to tests these I think.

[nogtini]

I think a brief mention at least somewhere in the Stimulus handbook addressing that it's preferred to use system tests (and no current path for unit tests) would bode well for transparency. Even just a brief mention in something like testing support after browser support would be helpful.