bpo-34170: _PyCoreConfig_Read() defaults to argc=0 (GH-8595)

Add unit tests for argc and argv of _PyCoreConfig.
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 79622f1..25593bd 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -272,6 +272,8 @@
 
         'pycache_prefix': '(null)',
         'program_name': './_testembed',
+        'argc': 0,
+        'argv': '[]',
         'program': '(null)',
 
         'isolated': 0,