chore(vue3): add extra ticks in dropdown tests

What does this MR do?

Vue3 changed "internal" execution model - when watchers / lifecycle hooks are started one tick later

In order to support our future migration to Vue3 I need to do this ugly as hell MR - just adding next ticks

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.