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.
By default we generate human-readable diagnostics on the assumption that
the caller is a simple program that is capturing stdin via a pipe and
letting stderr go to the terminal.
More sophisticated callers may wish to analyze the diagnostics themselves
and perhaps present them in a different way, such as via a GUI.
This option skips the usual decoding step and instead prints out a JSON-
formatted list of the variables that are referenced by the configuration.
In simple cases this is not required, but for more complex use-cases it
can be useful to first analyze the input to see which variables need to
be in the scope, then construct a suitable set of variables before finally
decoding the input. For example, some of the variable values may be
expensive to produce.
This is essentially a CLI wrapper around the hcldec package, accepting a
decoding specification via a HCL-based language and using it to translate
input HCL files into JSON values while performing basic structural and
type validation of the input files.
