Fix tests to prepare for @vue/test-utils update

What does this MR do?

This MR updates our tests so @vue/test-utils could be updated to 1.0.beta.30. This suite is also compatible with our current version of @vue/test-utils, so we have an opportunity to review changes in our tests separately to version updates & snapshot updates.

Most of the changes are related to:

• setData, setProps and other things require nextTick to update component
• stubbing of components now works "correctly" - so when you do {stubs: GlTable} and use shallowMount - GlTable is executed, but underlying component BTable will be stubbed in .beta.30, which is not the case for our version - for such tests shallowMount was replaced by mount

Checklist

• [n/a] Changelog entry
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] Separation of EE specific content