Improve testing of validation

[yarikoptic]

Follow up to discoveries in

• Overhauling validation results to get them closer to cover different types of validators #1514

TODOs

• test validation of a zarr (valid and invalid) since https://github.com/dandi/dandi-cli/pull/1514/files#diff-2fe358926b67bf5e55ea39d873932575efecdaf2c1cf1facace7aa0259cb4c1cR75 not covered
• test validation on failing validation nwb -- seems https://github.com/dandi/dandi-cli/pull/1514/files#diff-222d83ee68f525e4cf9e4b6b189b26401c17338b7b40c0d65b46e66968226c71R430 not covered