Running PHPUnit test from CLI and debug in PhpStorm: Remote file path '/directory/path/-' is not mapped to any file in project
You can disable "Force break at first line when no path mapping specified" in "Debug" settings and this error goes away. While execution is paused, you can see that the break is happening in a secondary callback that is called with - as an argument (See $_SERVER['argv'][0]).

This doesn't happen with purely unit tests because there isn't a secondary callback but BrowserTestBase. KernelTestBase must be working simillarly to BrowserTestBase.