Add a test_get_help_epilog test

tests/test_nbcourse.py

 from nbcourse.initialize import initialize
 from nbcourse.nbcourse import NbCourse, MyDoitMain, ClassTaskLoader
 from nbcourse.utils import get_functions
+from nbcourse import get_help_epilog
 import pytest
 from pathlib import Path
 import os
@@ -11,6 +12,11 @@ TESTS_PATH = Path(__file__).parent.absolute()
 MINI_CONF = {'nb': {'dir': 'sample'}}
 
 
+def test_get_help_epilog():
+    """Test get_help_epilog function for CLI"""
+    get_help_epilog()
+
+
 @pytest.fixture
 def create_project(tmpdir):
     """Create a sample nbcourse project"""