bpo-33053: -m now adds *starting* directory to sys.path (GH-6231) (#6236)

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.

(cherry picked from commit d5d9e02dd3)

Lib/test/support/script_helper.py

@@ -87,6 +87,7 @@ class _PythonRunResult(collections.namedtuple("_PythonRunResult",
 # Executing the interpreter in a subprocess
 def run_python_until_end(*args, **env_vars):
     env_required = interpreter_requires_environment()
+    cwd = env_vars.pop('__cwd', None)
     if '__isolated' in env_vars:
         isolated = env_vars.pop('__isolated')
     else:
@@ -125,7 +126,7 @@ def run_python_until_end(*args, **env_vars):
     cmd_line.extend(args)
     proc = subprocess.Popen(cmd_line, stdin=subprocess.PIPE,
                          stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                         env=env)
+                         env=env, cwd=cwd)
     with proc:
         try:
             out, err = proc.communicate()