There is a convention in Moodle code that, for example, the unit tests for lib/moodlelib.php are in lib/tests/moodlelib_test.php.

For enrollib.php, lib/tests/moodlelib_test.php does not exist. Instead, there are some tests in lib/tests/accesslib_test.php and others in enrol/tests/enrollib_test.php. This should be cleaned up.