Add Validator test for `check_missing_date_files`

[krivard]

Before #1749, the validator test case for no output files aggravated the check_missing_date_files validation failure. Now, it correctly aggravates the check_empty_filelist instead, but that means we no longer have coverage for check_missing_date_files. It would be good to add a test that verifies we can detect missing date files among a non-empty collection of output files.

[krivard]

Completed as part of #1749