fix: make scoped slots rendering consistent for stubs

[xanf]

This MR fixes extremely weird stubbing behavior. Currently scoped slot stubs will be rendered only for components, which are located at root level of other component. Basically:

<some-cmp>foo</some-cmp>

will have scoped slots rendered on stub, while

<div><some-cmp>foo</some-cmp>

will have only default slot rendered.
This is caused because slot lookup was performed on parent instead of instance itself

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

This is definitely a breaking change: now we try to render more scoped slots than before. Basically, that means that some tests which rely that slots were not previously rendered will fail. However, I see this definitely a better thing than current confusing approach

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

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[xanf]

This try catch is best-effort attempt to mitigate migration from existing state. Since we're now rendering more scoped slots on stubs than previously there is higher chance of blow up.

While it might create some confusion (hey, some my slots are rendering, some are not) considering close EOL of Vue.js 2 I'm thinking of this one as acceptable trade-off

For example at GitLab codebase, without try - catch this MR entirely blows over 700 tests, with try catch - less than 100, however I'm open for discussion on removing it

[lmiller1990]

Sorry, just saw this one @xanf. I'll review it now.