Starting with a new machine, and a new buildkit instance! Buildkit/bin's contents added to PATH successfully.
When I run cv php:boot --level classloader on the command line, I get:
/*BEGINPHP*/ require_once '/var/www/project/sites/all/modules/civicrm' . "/CRM/Core/ClassLoader.php";\CRM_Core_ClassLoader::singleton()->register(); /*ENDPHP*/
But when phpunit tests are through the command line (with any of phpunit5, phpunit6, or phpunit7), the cv command line fails, returning "".
In the test directory's phpunit bootstrap file, replacing $result with what's output above works until the next time cv is called in the process, whereupon it also breaks.
I have set the execute permission on ~/buildkit/bin/cv to be globally executable. But it's the same user running it that cloned the buildkit repo.