internal: Better testing infra for ratoml
This PR makes some improvements on how we test configs that come from `rust-analyzer.toml` files.
It was primarily used to solve #18021 but along the way I could not really determine the cause of the said issue which makes me think that it may not be related to the changes that I made earlier to the ratoml infra. In either way `custom_snippets` are now made `global` because we still don't have a tree that maps a `SourceRootId` to a set of `Snippet`s.
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