gh-157006: Clear PYTHONSTARTUP and other PYTHON* env vars in test_embed

run_embedded_interpreter() now defaults to remove_python_envvars()
when no explicit env is provided, preventing PYTHONSTARTUP (e.g.,
set by VS Code shell integration) from leaking into the embedded
interpreter and causing test failures.

Also fix test_init_run_main_startup_exitcode to use
remove_python_envvars() instead of dict(os.environ) as the base
environment.

Author: don012rac3

Lib/test/test_embed.py

@@ -137,6 +137,8 @@ def run_embedded_interpreter(self, *args, env=None,
         """Runs a test in the embedded interpreter"""
         cmd = [self.test_exe]
         cmd.extend(args)
+        if env is None:
+            env = remove_python_envvars()
         if env is not None and MS_WINDOWS:
             # Windows requires at least the SYSTEMROOT environment variable to
             # start Python.
@@ -628,7 +630,7 @@ def test_init_run_main_startup_exitcode(self):
             with open(filename, 'x') as fp:
                 fp.write(CODE_EXITCODE_123)
 
-            env = dict(os.environ)
+            env = remove_python_envvars()
             env['PYTHONSTARTUP'] = filename
             self.check_program_exitcode("test_init_run_main_interactive_exitcode",
                                         env=env,

Misc/NEWS.d/next/Tests/2026-09-06-12-00-00.gh-issue-157006.Xk9mPq.rst

@@ -0,0 +1,4 @@
+Fix ``test.test_embed`` to clear ``PYTHON*`` environment variables (including
+``PYTHONSTARTUP``) before running embedded interpreter tests, preventing failures
+when the test runner's environment has these variables set (e.g., by VS Code's
+shell integration).