In a clean l3build repository,

• executing texlua ./l3build.lua check undefined-test, the last lines of stdout are
  • before

    Running checks on
      undefined-test (1/1)
    Failed to find input for test undefined-test
              --> failed
    
    
      Check failed with difference files
    
    

  • after

    Failed to find input for test undefined-test
              --> failed
    
    Check failed with no difference files.
    
    

• executing texlua ./l3build.lua check --rerun undefined-test, the last lines of stdout are
  • before

    Failed tests for configuration "build":
    
      Check failed with difference files
    
    Failed tests for configuration "config-pdf":
    
      Check failed with difference files
    
    Failed tests for configuration "config-plain":
    
      Check failed with difference files
    
    Failed tests for configuration "config-context":
    
      Check failed with difference files
    
    

  • after

    Check failed for configuration "build" with no difference files.
    
    Check failed for configuration "config-pdf" with no difference files.
    
    Check failed for configuration "config-plain" with no difference files.
    
    Check failed for configuration "config-context" with no difference files.