There are plenty of unit/regression testing tools for Emacs, and even some for functional testing. What Emacs is missing though is a really good testing framework for integration testing. This is where Ecukes comes in.
Cucumber is a great integration testing tool, used mostly for testing web applications. Ecukes is Cucumber for Emacs. No, it's not a major mode to edit feature files. It is a package that makes it possible to write Cucumber like tests for your Emacs packages.
Ecukes is not a complete clone of Cucumber and is not intended to be. If however Ecukes is missing some feature that you feel really should be included, please make a bug report.
If you don't know anything about Cucumber I suggest you read up a bit about it before continuing with Ecukes.
See http://ecukes.info for more information!