(fix #8337) test(component-slot): new child vnode gets destroyed instead of old one

[privatenumber]

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

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master 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/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:
I'm not confident in the quality of the fix (whether it's addressing the bug directly or indirectly) but deep-cloning the vnode in createElm when a vnode used in a previous render is encountered seems to fix the bug.

In any case, a test has been added to repro the bug.

[privatenumber]

Not sure what's up with the failed lint-flow-type test since I didn't touch the file it's throwing an error at

Edit: Looks like this direct commit bypassed the test