Which one of these is the correct one?
- The registers testcase checks that the module's register interface works as expected.
- The registers testcase checks that the module's register interface is working as expected.
"Working" is a present participle.
According to this article, "A Present Participle is used with the verb 'To Be' to indicate an action that is incomplete".
Therefore, if the register interface is a completed action, the first sentence is the better choice. If not, then the second sentence is correct.
Personally, I would say the first sentence is better, because you're checking the condition that it completes the action. If it works, the test succeeds. If it doesn't, the test fails.
(Btw, this question is probably better asked here: https://english.stackexchange.com/).