fix: use Vue async option for sync mode

[eddyerburgh]

This PR fixes many bugs caused by the current synchronous implementation by using an async option added to Vue 2.5.18.

• closes Fix synchronous behavior #676
• fixes RangeError: Maximum call stack size exceeded when use vue-i18n and i18n large json #1125
• fixes setProps performance issues leads to RangeError: Maximum call stack size exceeded #1046
• fixes Re-render does not occur synchronously when reactive data is changed in mounted life-cycle hook #1035
• fixes False positive prop mutation error #1034
• fixes Vuelidate Error in render: “TypeError: Cannot read property 'proxy' of undefined” #1023
• fixes "Cannot read property '_transitionClasses' of undefined" when injecting vee-validate which is not used #829 (vue-test-utils > 1.0.0-beta.12 fails with vee-validate logaretm/vee-validate#1267)
• fixes Can't trigger a second click on a Vuetify checkbox. #594
• fixes $listeners is readonly error #532

Note:

• Causes new bug in this.$children is empty in the created lifecycle hook #827

This change introduces a bug shown in this fiddle. Since this is an unusual usage of $children we have decided to move forward with this PR. However, there may be other edge cases. If there are many more edge cases, then we will remove sync functionality from Vue Test Utils.

[38elements]

It may be necessary to revert #675.

[eddyerburgh]

For the moment we will continue to run sync compat mode (for Vue < 2.5.18), so that change is still
required :)

[38elements]

I think it is necessary to change .circleci/config.yml.

[ryanlitalien]

Following along, is there anything else that needs to be added for this to get merged? Thanks!

[agualis]

💪🤗