tests: Use en_US.UTF-8 locale when C.UTF-8 not available

Some operating systems don't have the `C.UTF-8` locale installed yet
(for instance, CentOS 7). In such a case, fallback to `en_US.UTF-8` so
that tests can be run.

This follows commit 92ff315.

tests/test_cli.py

@@ -299,7 +299,10 @@ class CommandLineTestCase(unittest.TestCase):
         # Make sure the default localization conditions on this "system"
         # support UTF-8 encoding.
         loc = locale.getlocale()
-        locale.setlocale(locale.LC_ALL, 'C.UTF-8')
+        try:
+            locale.setlocale(locale.LC_ALL, 'C.UTF-8')
+        except locale.Error:
+            locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
 
         sys.stdout, sys.stderr = StringIO(), StringIO()
         with self.assertRaises(SystemExit) as ctx: