[1.7.x] Made model_regress unpickling test CWD-independent

Refs #24007. Thanks Tim Graham for his help with the patch.

Backport of 1d9fc5caa9 and
995be4a137 from master

tests/model_regress/tests.py

@@ -2,13 +2,14 @@ from __future__ import unicode_literals
 
 import datetime
 from operator import attrgetter
+import os
 import pickle
 import subprocess
 import sys
-import tempfile
 import unittest
 
 from django.core.exceptions import ValidationError
+from django.core.files.temp import NamedTemporaryFile
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import six
 from django.utils.timezone import get_fixed_timezone
@@ -272,11 +273,19 @@ print(article.headline)"""
             article_text="This is an article",
         )
 
-        with tempfile.NamedTemporaryFile(mode='w+', suffix=".py", dir='.', delete=True) as script:
+        with NamedTemporaryFile(mode='w+', suffix=".py", dir='.') as script:
             script.write(script_template % pickle.dumps(a))
             script.flush()
             try:
-                result = subprocess.check_output(['python', script.name])
+                result = subprocess.check_output(
+                    [sys.executable, script.name],
+                    env={
+                        # Needed to run test outside of tests directory
+                        str('PYTHONPATH'): os.pathsep.join(sys.path),
+                        # Needed on Windows because http://bugs.python.org/issue8557
+                        str('PATH'): os.environ['PATH'],
+                    }
+                )
             except subprocess.CalledProcessError:
                 self.fail("Unable to reload model pickled data")
         self.assertEqual(result.strip().decode(), "Some object")