Fix memory leak and potential props clash

[mcous]

Overview

As discussed in person, there's a lot of overlap between @threlte/test and the vanilla @testing-library/svelte. As I was exploring integrating the two libraries more closely via a peer dependency relationship, a couple little issues got in my way that I figured I'd put a PR up for

Change log

• Remove memory leak of canvas elements getting added to a Set but never removed
  • There didn't appear to be any actual cleanup needed, so I removed the Set
• Ensure component props can't class with container props by creating an explicitly named componentProps prop for the container
  • This is easier to read in the code than it is to describe in a bullet point 🫠
• Fix a bad validity check that would've allowed an array to masquerade as an object

[changeset-bot]

🦋 Changeset detected

Latest commit: 21bdb2b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@threlte/test	Patch

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[michealparks]

Great stuff as always, thank you!