refactor(test): add new APIs for easier snapshot testing

[rami3l]

Borrowing from Cargo's previous effort, this marks the beginning of a new testing DSL for better overall dev experience.

RA-powered inline snapshot updates seems to work without issues in VSCode with the given demo:

I have a more ambitious goal of removing dependency injections altogether, but we must start from somewhere, so here it is.

Concerns

• Could the API design use some improvements (naming, signature, ...)?
• Should we start to mark the old APIs as deprecated as Cargo once did with theirs?
• What redactions should we include by default (this_host_triple, tempdir, ...)?
  • This can be added as we go; it does look like this_host_triple is a good inclusion at the start; and I've made it so that each test can add their own redactions.

[ChrisDenton]

I really like this. Using a more principled testing framework makes a lot of sense rather than maintaining a bespoke solution. And it does seem to be a mostly drop-in replacement but allows for more exact matching (no need for contains) and easier updating of expected output when it's intentionally changed.

It'd be great if we can figure out how to make the old APIs deprecated while still making CI pass but I think this is worth doing even if that proves challenging.

I don't have a strong opinion on naming. Maybe "redactions" could be "replacements"? Because it replaces the matched string.

[rami3l]

@ChrisDenton Many thanks for the kind words! Redaction is snapbox terminology so I'll probably stick with that...

My plan is that we can merge this as-is first (with #![allow(deprecated)]), and then move on with the migration one bit at a time.

[djc]

Looks like a good direction!

[djc]

Nit: move these below the module-level docstrings, please?

[djc]

I don't love this because I usually expect str to be std::str. Maybe alias this or just use snapbox::str!() in callers?