Add pytest-subtests files changes

In addition, enable the plugin in `pytest/__init__.py` and `config/__init__.py`.

Author: nicoddemus

src/_pytest/config/__init__.py

@@ -271,6 +271,7 @@ def directory_arg(path: str, optname: str) -> str:
     "logging",
     "reports",
     "faulthandler",
+    "subtests",
 )
 
 builtin_plugins = {