bpo-30078: Add an example of passing a path to unittest (GH-1178)

(cherry picked from commit f7e62cf8)

Lib/unittest/main.py

@@ -14,6 +14,7 @@ Examples:
   %(prog)s test_module               - run tests from test_module
   %(prog)s module.TestClass          - run tests from module.TestClass
   %(prog)s module.Class.test_method  - run specified test method
+  %(prog)s path/to/test_file.py      - run tests from test_file.py
 """
 
 MODULE_EXAMPLES = """\