Document absoluteness of sys.executable

Closes #13402.
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 6c5fc24..6e37ac4 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -471,6 +471,9 @@
         self.assertRaises(TypeError, sys.call_tracing, str, 2)
 
     def test_executable(self):
+        # sys.executable should be absolute
+        self.assertEqual(os.path.abspath(sys.executable), sys.executable)
+
         # Issue #7774: Ensure that sys.executable is an empty string if argv[0]
         # has been set to an non existent program name and Python is unable to
         # retrieve the real program name