add a preprocess test for other configs

[lsf37]

For some PR's it'd be nice to know that only specific configurations change (e.g. visible for specific boards or architectures only, or for SMP only, etc).

We could achieve this with a preprocess test for all configs that reports which ones changed and optionally shows the AST and code diffs. Output could include a comment on the PR with a link to the diff for any changed configs.

This would likely have to be optional (e.g. requested by label), since we'd expect to almost always change something in at least one config.