bpo-36142: Add _PyMem_GetDebugAllocatorsName() (GH-12185)

The development mode now uses the effective name of the debug memory
allocator ("pymalloc_debug" or "malloc_debug"). So the name doesn't
change after setting the memory allocator.
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 1f236a9..2827e87 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -336,6 +336,7 @@
             'legacy_windows_fs_encoding': 0,
             'legacy_windows_stdio': 0,
         })
+    DEBUG_ALLOCATOR = 'pymalloc_debug' if support.with_pymalloc() else 'malloc_debug'
 
     # main config
     COPY_MAIN_CONFIG = (
@@ -588,7 +589,7 @@
 
     def test_init_env_dev_mode(self):
         config = dict(self.INIT_ENV_CONFIG,
-                      allocator='debug',
+                      allocator=self.DEBUG_ALLOCATOR,
                       dev_mode=1)
         self.check_config("init_env_dev_mode", config)
 
@@ -596,7 +597,7 @@
         config = {
             'dev_mode': 1,
             'faulthandler': 1,
-            'allocator': 'debug',
+            'allocator': self.DEBUG_ALLOCATOR,
         }
         self.check_config("init_dev_mode", config)