Local test failure - BigPipeTest

Problem/Motivation

I ran BigPipeTest locally and got

Exception: Exception: You are not allowed to say llamas are not cool!
Drupal\big_pipe_test\BigPipeTestController::exception()() (Line: 197)

Thought this might be just my local for some reason but @nicxvan and @godotislate both got the same error when they tried it.

Also tried with an unedited clean phpunit.xml.dist copied over what I usually use, and got the same error with that.

I assume this is something to do with phpunit error handling, but don't remember anything specific about that changing.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet