Hmm, I tried to add a test but it's not really possible.

When can it happen that this method does not exist?

Ah yes I removed the test. but it is still in the PR title and commit message ;)
Try to throw an exception in the setUpBeforeClass and you will see.
(Note I did not dig in PHPUnit to find the root cause. I'm not sure I want to :D )

Could it be related to $test being an instance of TestSuite but not TestCase?

I don't know. What do you have in mind ?

We could then use instanceof instead.

I was pretty sure you are going to say that.
using instanceof will be a bit boring because we manage many PHPUnit version (pear convention vs namespace :/)

There is an alias for the namespaced class in PHPUnit 4.8 and 5.x. :)

@xabbuh It should be ok

Thank you @lyrixx.