WordPress plugin testing unit test or acceptance test or both
(Ian pinged me on Twitter and asked me to provide some feedback)
First, I will say that I have not used the combo of PHPUnit and Selenium for doing acceptance testing. I use Behat + Mink to do that, but that's not the issue here I think.
To test a login form, you could do unit tests AND acceptance tests. It's just a question of whether or not you feel it's valuable to do both. I think from the perspective of building a WordPress plugin, you will get a much better return on the time creating the tests to go with acceptance tests.
Some other answers:
- Unit testing stuff that spits out HTML is totally acceptable. After all, the goal is to make sure that individual module of code works correctl
- Testing that error messages on failed logins occur should be in it a separate test. These days, I like to practice "one assertion per test" at the unit level
Hope this helps!