Issue #13415: Skip test_os.test_unset_error on FreeBSD < 7 and OS X < 10.6

(where unsetenv() doesn't return a value).

Lib/test/test_os.py

@@ -447,6 +447,10 @@ class EnvironTests(mapping_tests.BasicTestMappingProtocol):
         value_str = value.decode(sys.getfilesystemencoding(), 'surrogateescape')
         self.assertEqual(os.environ['bytes'], value_str)
 
+    # On FreeBSD < 7 and OS X < 10.6, unsetenv() doesn't return a value (issue
+    # #13415).
+    @support.requires_freebsd_version(7)
+    @support.requires_mac_ver(10, 6)
     def test_unset_error(self):
         if sys.platform == "win32":
             # an environment variable is limited to 32,767 characters