When we did the automatic rewriting from the hashicorp/hcl2 import paths
to hashicorp/hcl/v2 import paths, our automatic rewrite script did not
run "go fmt" afterwards and so left some imports misordered as a result
of the slightly-differently-shaped directory structure in the new
repository.
The harness can now run tests that decode successfully and compare the
result with a given value. Further work is required in later commits to
deal with other cases, such as tests that intentionally produce errors.
