Fixed #26677 -- Converted some i18n tests to use disposable FS tree.

This allows makemessages/compilemessages tests in `test_extraction.py`
and `test_compilation.py` to actually run isolated from each other
(unaffected by stray FS objects left by cleanup actions failures, debug
sessions, etc.) and to take advantage of the parallel tests execution
feature like most of the Django test suite.

`test_percents.py` gets slightly refactored to not inherit from the new
machinery which sets up every test case to copy and run under a
temporary tree.

tests/i18n/test_compilation.py

@@ -5,6 +5,7 @@ import gettext as gettext_module
 import os
 import shutil
 import stat
+import tempfile
 import unittest
 from subprocess import Popen
 
@@ -23,15 +24,25 @@ from django.utils.six import StringIO
 from django.utils.translation import ugettext
 
 has_msgfmt = find_command('msgfmt')
+source_code_dir = os.path.dirname(upath(__file__))
 
 
 @unittest.skipUnless(has_msgfmt, 'msgfmt is mandatory for compilation tests')
 class MessageCompilationTests(SimpleTestCase):
 
-    test_dir = os.path.abspath(os.path.join(os.path.dirname(upath(__file__)), 'commands'))
+    work_subdir = 'commands'
 
     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))
+        shutil.copytree(os.path.join(source_code_dir, self.work_subdir), self.test_dir)
+        # Make sure we step out of the temporary working tree before we
+        # remove it as we might be pulling the rug from under our own feet
+        # othewise. Rhis is especially true on Windows.
+        # Remember cleanup actions registered with addCleanup() are called in
+        # reverse so this ordering is important:
+        self.addCleanup(self._rmrf, self.test_dir)
         self.addCleanup(os.chdir, self._cwd)
         os.chdir(self.test_dir)
 
@@ -114,13 +125,12 @@ class MultipleLocaleCompilationTests(MessageCompilationTests):
 
 class ExcludedLocaleCompilationTests(MessageCompilationTests):
 
-    test_dir = os.path.abspath(os.path.join(os.path.dirname(upath(__file__)), 'exclude'))
+    work_subdir = 'exclude'
 
     MO_FILE = 'locale/%s/LC_MESSAGES/django.mo'
 
     def setUp(self):
         super(ExcludedLocaleCompilationTests, self).setUp()
-
         shutil.copytree('canned_locale', 'locale')
         self.addCleanup(self._rmrf, os.path.join(self.test_dir, 'locale'))