Refactor tests: isolate test using errors.Unwrap

[dolmen]

Isolate TestWrappedError_IsCompatibleWithErrorsUnwrap which uses errors.Unwrap (which appeared with Go 1.13) in a separate test source compiled only for Go 1.13+.

Note: other tests that use %w with fmt.Errorf should be isolated too. I will provide those changes once this PR is accepted.

[dolmen]

Cc: @dnephin for review as you contributed #9

[dnephin]

Hello, thank you for the PR! I am no longer a maintainer on this project, so I can't give you an official position.

If I understand correctly, you are making this change to support Go versions earlier than go1.13. The Go team only supports the latest two versions (Go1.21, Go1.22), and most libraries follow the same practice. go1.12 is sufficiently old now that I would say moving the code to a separate file is not worth it.

[dolmen]

The project has no Go version mentioned in go.mod, so there is no official stance about the minimum Go version supported.

This project still exists to support downstream projects which haven't yet fully embraced the improved error interface from go1.13 and go1.20. Newer code should move away from it.
So nowadays maintaining this project is only relevant to keep running older Go code that might be running on recent go versions or not.

I'm a candidate for being a co-maintainer if necessary (FYI I'm also the current lead maintainer of Testify, if that matters regarding maintaining an old codebase).

[dolmen]

(Note that I'm also supporting the effort to detach go-multierror from errwrap: hashicorp/go-multierror#60)