style: prefer crate-specific error where possible

[jmayclin]

Goal

Use a more specific error type.

Why

We have confusing error types that we use in our testing harnesses.

type Error = Box<dyn std::error::Error>;
type Result<T, E = Error> = core::result::Result<T, E>;

These were then referred to as crate::testing::Error, etc. This was confusing because you then expect them to be test specific errors but they were just wrapped around standard library types.

More specifically, the Box<dyn std::error::Error> is very rarely necessary in our tests, because everything returns a crate::error::Error. We should prefer to use the more description/specific error type as return values for our tests.

How

All tests (except 2) only ever return the crate specific error type. I just used unwraps for the two tests that were working with IO errors.

Testing

Existing CI should pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.