Hard to review snapshots because they are far from their intended code

[epage]

Problem

Right now we have

tests/snapshots/add/*
...
tests/testsuite/cargo_add.rs
...

This is mildly annoying to review in small cases but in cases like #10620, its a big pain

Steps

See #10620

Possible Solution(s)

In cargo-test-macro we add a macro that will discover test files in the snapshot directory

To ensure it rebuilds when new tests are added, we add a build.rs that says to rebuild when that directory changes (downside being we'll rebuild more when making test changes which avoiding that was one of the benefits of snapshots)

Notes

We should wait for #10620 so we can update all of them at once

Version

No response

[Muscraft]

This should also wait for #10626 to be resolved, as it will reorganize snapshot tests