Definition result of the extensiontest is differ
From MediaWiki extension testing
When a Wikiation_exttest.py runs a test on a testcase the possible can be :
Result : Differ
The result of the test is the Differ as expected. This is allmost equal to "Test failed"
If the behaviour of the extension should not be changed since the last test then this IS "test failed". But if the behaviour of the extension should result in a different output then this might be ok.
Differ is something a program can measure. A program can not give a value of right or wrong, only a human can do so. A human can decide that Differ means "test failed". But that is often very easy to after the test is run.