Error message misdirection with incorrect top-level key

[nicholas-sly]

When creating a new test config, I misnamed the doc key at the top level of the subtest as description. This resulted in a test config error along the lines of Config element 'module_wrappers.*' is expected to be a dict/mapping, but got 'blah' where 'blah' was the value for the first key, despite that key being valid.

As an example:

_base:
  summary: blah
  subtitle: blahblah
  description: |
    blahblahblah

This config would result in the error above.

The expected behavior is for the test config error to inform me that description is not a valid key for the top-level of a subtest config. Optionally, how to get a list of valid top-level keys on the command line would be nice as well, but not strictly required.

The most recent commit hash where this was tested was 78a9a0beb94ddc0a7c85314f9ed71f34a8eb09a8.