This begins updates the filesystem layout for the tests to match
the new layout from the standard test suite (See:
https://github.com/dhall-lang/dhall-lang/pull/265)
This is still missing a part of the standard requirement, which is checking
parsing and import test results against an expected output (instead of
just checking that they succeed). I plan to add that in a subsequent
pull request. This is mainly to unblock other features that require using
the new standard layout.
The motivation for this change is:
* To catch build failures in downstream packages whenever we make a breaking
change to the `dhall` API
* To reduce the amount of work I need in order to cut a release for all of
these packages
* To better share Nix/CI-related logic between the projects
Note that I have not yet migrated `dhall-nix` in. I'm waiting for
https://github.com/dhall-lang/dhall-nix/issues/17 to be fixed since
`dhall-nix` is incompatible with later versions of `megaparsec` due to
`hnix`.