Fix my test introduced in test_sys by r79394:

Restore the orginal filesystem encoding before testing
assertRaises(LookupError, sys.setfilesystemencoding, "xxx"). Unittest formats
the exception, but the formatting failed because the file system was invalid
(set to iso-8859-1 by the previous test).

Anyway, ensure to restore the original filesystem encoding when exiting
test_setfilesystemencoding() to avoid error propagation to the other tests.

Lib/test/test_sys.py

@@ -797,9 +797,14 @@ class SizeofTest(unittest.TestCase):
 
     def test_setfilesystemencoding(self):
         old = sys.getfilesystemencoding()
+        try:
             sys.setfilesystemencoding("iso-8859-1")
             self.assertEqual(sys.getfilesystemencoding(), "iso-8859-1")
+        finally:
+            sys.setfilesystemencoding(old)
+        try:
             self.assertRaises(LookupError, sys.setfilesystemencoding, "xxx")
+        finally:
             sys.setfilesystemencoding(old)
 
 def test_main():