* Refactor JUnit XML output to use new concept of an Artifact
* Move JUnit-related code into new `artifact` package
* Refactor Artifact's Save method to return diagnostics, update comments
Previously TestJUnitXMLFile implemented the View interface, which cannot return errors. Now it's not a View any more we can simplify things.
* Make junitXMLTestReport output deterministic by iterating over a slice instead of a map, add test
* Provide sources to junitXMLTestReport, allowing complete error messages in the XML
We need to ensure that artifact.NewTestJUnitXMLFile is called once the config Loader is available as a non-nil pointer
* Whitespace
* Add some test coverage for JUnit XML output for `terraform test`
* Refactor how file is saved, add tests
* Move XML structs definitions outside of `junitXMLTestReport`
* Fix nil pointer bug
* Add missing file headers
* Refactor comparison of byte slices
* Rename package to `junit`, rename structs to match
* Add a test showing JUnit output when a test is skipped by the user
