The entire test suite can be run by simply entering the test directory and typing phpunit:
cd /path/to/joomla/tests/unit phpunit
You can run a subset of tests by specifying the directory path of the tests that you want to run.
Will run all of the tests in the specified directory and any subdirectory.
The --filter parameter allows you to run a specific subset of the entire test suite. The --filter parameter takes a regular expression that can be used to filter out tests.
phpunit --filter JForm
Will scan through the suite directory and run tests that contain JForm in the class name or in the method name.