Refs #26677 -- Simplified i18n test cleanups.

The fact that we aren't dealing with the Django source tree anymore
allows us to drop several tearDown()/addCleanup() calls that were
concerned with removing apiece files/dirs/symlinks created by test
cases, as we are covered by the removal of the parent temporary tree
anyways.

Thanks Tim Graham for advice and review.

tests/i18n/utils.py

@@ -0,0 +1,67 @@
+import os
+import re
+import shutil
+import tempfile
+
+from django.utils._os import upath
+
+
+source_code_dir = os.path.dirname(upath(__file__))
+
+
+def copytree(src, dst):
+    shutil.copytree(src, dst, ignore=shutil.ignore_patterns('*.pyc', '__pycache__'))
+
+
+class POFileAssertionMixin(object):
+
+    def _assertPoKeyword(self, keyword, expected_value, haystack, use_quotes=True):
+        q = '"'
+        if use_quotes:
+            expected_value = '"%s"' % expected_value
+            q = "'"
+        needle = '%s %s' % (keyword, expected_value)
+        expected_value = re.escape(expected_value)
+        return self.assertTrue(
+            re.search('^%s %s' % (keyword, expected_value), haystack, re.MULTILINE),
+            'Could not find %(q)s%(n)s%(q)s in generated PO file' % {'n': needle, 'q': q}
+        )
+
+    def assertMsgId(self, msgid, haystack, use_quotes=True):
+        return self._assertPoKeyword('msgid', msgid, haystack, use_quotes=use_quotes)
+
+
+class RunInTmpDirMixin(object):
+    """
+    Allow i18n tests that need to generate .po/.mo files to run in an isolated
+    temporary filesystem tree created by tempfile.mkdtemp() that contains a
+    clean copy of the relevant test code.
+
+    Test classes using this mixin need to define a `work_subdir` attribute
+    which designates the subdir under `tests/i18n/` that will be copied to the
+    temporary tree from which its test cases will run.
+
+    The setUp() method sets the current working dir to the temporary tree.
+    It'll be removed when cleaning up.
+    """
+
+    def setUp(self):
+        self._cwd = os.getcwd()
+        self.work_dir = tempfile.mkdtemp(prefix='i18n_')
+        self.test_dir = os.path.abspath(os.path.join(self.work_dir, self.work_subdir))
+        copytree(os.path.join(source_code_dir, self.work_subdir), self.test_dir)
+        # Step out of the temporary working tree before removing it to avoid
+        # deletion problems on Windows. Cleanup actions registered with
+        # addCleanup() are called in reverse so preserve this ordering.
+        self.addCleanup(self._rmrf, self.test_dir)
+        self.addCleanup(os.chdir, self._cwd)
+        os.chdir(self.test_dir)
+
+    def _rmrf(self, dname):
+        if os.path.commonprefix([self.test_dir, os.path.abspath(dname)]) != self.test_dir:
+            return
+        shutil.rmtree(dname)
+
+    def rmfile(self, filepath):
+        if os.path.exists(filepath):
+            os.remove(filepath)