Auto merge of #10581 - epage:snapbox, r=ehuss

Integrate snapbox in with cargo-test-support

### What does this PR try to resolve?

#10472 introduced snapbox to cargo's tests in the least intrusive manner by copying some cargo-test-support code.  Primarily, this PR works to de-duplicate that code.  Secondarily, it makes it possible for snapbox to be used by other cargo tests that can work with its more limited functionality compared to cargo-test-support.

### How should we test and review this PR?

This is broken down by commits for smaller chunks to look over with some extra details in some of the commit messages.

As this is effectively refactoring existing tests, them passing is sufficient for testing.  The main focus would be on any API design including if there are any practices that we used to do that this continues forward to snapbox that we shouldn't.

### Additional information

The cargo contributing guide also needs to be updated but I'm leaving that off for another PR once this is merged so we have a clearer idea of what the API will look like (less churn) and so we can focus the conversation for each PR.