dev: schema tests output reason if "pass" test fixture fails

[ralfhandl]

This is a quality-of-life improvement for writing "pass" test fixtures: output validation errors if a "pass" test fixture fails validation.

Tick one of the following options:

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[duncanbeevers]

This is a fine change, but I think the PR would be easier to review if you showed exactly what a failure looked like before, and what it looks like with this change applied.