bpo-33053: -m now adds *starting* directory to sys.path (GH-6231)

Historically, -m added the empty string as sys.path
zero, meaning it resolved imports against the current
working directory, the same way -c and the interactive
prompt do.

This changes the sys.path initialisation to add the
*starting* working directory as sys.path[0] instead,
such that changes to the working directory while the
program is running will have no effect on imports
when using the -m switch.
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 6041f52..0746fcf 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1332,8 +1332,8 @@
 .. function:: run_python_until_end(*args, **env_vars)
 
    Set up the environment based on *env_vars* for running the interpreter
-   in a subprocess.  The values can include ``__isolated``, ``__cleavenv``,
-   and ``TERM``.
+   in a subprocess.  The values can include ``__isolated``, ``__cleanenv``,
+   ``__cwd``, and ``TERM``.
 
 
 .. function:: assert_python_ok(*args, **env_vars)