Add JS spec for squash and merge widget

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

gitlab-ce!18956 brings the squash and merge feature to CE.

Now there is no any JS spec for this Vue widget.

Some specs should be added to spec/javascripts/vue_mr_widget/components/states.

A possible name is mr_widget_squash_before_merge_spec.js as @kushalpandya suggested.