Refactor ready to merge widget tests to Vue test utils

spec/frontend/vue_mr_widget/components/states/__snapshots__/mr_widget_ready_to_merge_spec.js.snap

+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`ReadyToMerge with a mismatched SHA warns the user to refresh to review 1`] = `"<gl-sprintf-stub message=\\"New changes were added. %{linkStart}Reload the page to review them%{linkEnd}\\"></gl-sprintf-stub>"`;