[2.2.x] Fixed forms_tests.tests.test_renderers with Jinja 3.1.0+.

See https://github.com/pallets/jinja/pull/1621.
Backport of 1d9d082acf6e152c06833bb9698f88d688b95e40 from main

tests/forms_tests/tests/test_renderers.py

@@ -1,10 +1,12 @@
 import os
+import posixpath
 import unittest
 
 from django.forms.renderers import (
     BaseRenderer, DjangoTemplates, Jinja2, TemplatesSetting,
 )
 from django.test import SimpleTestCase
+from django.utils.version import get_version_tuple
 
 try:
     import jinja2
@@ -46,6 +48,28 @@ class Jinja2Tests(SharedTests, SimpleTestCase):
     renderer = Jinja2
     expected_widget_dir = 'jinja2'
 
+    @property
+    def jinja2_version(self):
+        return get_version_tuple(jinja2.__version__)
+
+    def test_installed_apps_template_found(self):
+        """Can find a custom template in INSTALLED_APPS."""
+        renderer = self.renderer()
+        # Found because forms_tests is .
+        tpl = renderer.get_template("forms_tests/custom_widget.html")
+        expected_path = os.path.abspath(
+            os.path.join(os.path.dirname(__file__), "..", self.expected_widget_dir)
+        )
+        if self.jinja2_version < (3, 1):
+            expected_path = os.path.join(
+                expected_path, "forms_tests", "custom_widget.html"
+            )
+        else:
+            expected_path = posixpath.join(
+                expected_path, "forms_tests", "custom_widget.html"
+            )
+        self.assertEqual(tpl.origin.name, expected_path)
+
 
 class TemplatesSettingTests(SharedTests, SimpleTestCase):
     renderer = TemplatesSetting