If there are no tests in a test class file, PHPUnit raises a warning (Event\Facade::emitter()->testRunnerTriggeredWarning() - 'no tests found in class') and will pass on a non-zero value to STDERR. Collision will not display this warning, but does pass on the STDERR value, which led to some confusion when it appeared that all tests were passing, but our CI pipeline was stopping at the test step in failure. However, narrowing it down to this issue took a while as at first I thought it was a bug in the CI runner.

You can recreate this with a default Laravel install:

composer create-project laravel/laravel empty-class
cd empty-class
php artisan test
# All tests pass
echo $?
# STDERR is 0
cat <(echo "<?php\nnamespace Tests\\\\Unit;\nuse PHPUnit\Framework\TestCase;\n\nclass EmptyClassTest extends TestCase{}") > tests/Unit/EmptyClassTest.php
php artisan test
# All tests pass
echo $?
# STDERR is 1
vendor/bin/phpunit
# Warnings!

It's not clear to me if this is limited to this one 'empty class' issue, or if all testRunnerTriggeredWarning events are ignored.