Move `idom.widgets.hotswap` to testing utils

[Archmonger]

Discussed in #865

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^{Originally posted by Archmonger December 30, 2022}
Ever since we've formalized support for conditionally rendered components, hotswap seems to have become a rather pointless utility.

This conversation is to discuss whether to deprecate/remove it.

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^{Originally posted by rmorshea December 30, 2022}
The primary function isn't really to facilitate conditional rendering. Rather, it's to allow you to swap components from outside the normal rendering flow. With that said, it's uses are pretty niche. At the moment it has one usage in IDOM's testing utilities. I'd say it would make sense to turn it into a private util in idom.testing.backend.