ert.el is a standard testing framework in Emacs which uses 'ert-deftest' to define test functions. Because it does not begin with 'def', it is not recognized as a definition by etags and libraries or packages with many tests will not have these show up for navigation when using default calls to etags.
I would suggest one of two solutions:
1. Modify etags to automatically recognize such definitions by default. (I tried using the --regex option but had little luck with that due to it not liking an opening paren without a closing one in the regex).
2. Add an alias of 'def-ert-test' and suggest people use that instead so that it is picked up by etags. #1 is obviously better given the number of existing test definitions that exist already.
Cheers,
-- rsw