Is run-tests a shell script?

The run-tests.sh isn't actually a shell script. It is a PHP script--which is why you must execute it with PHP. In fact, within core/scripts, each file is a PHP script file meant to be executed using the command line. These scripts are not intended to be run through a web server, which is one of the reasons for the .sh extension.

There can be issues with PHP across platforms that prevent providing a shebang line to allow executing the file as a normal bash or bat script. For more information, refer to this Drupal.org issue at https://www.drupal.org/node/655178.