test: xfail locale-gen option completion if --help is not available

At least on Debian 10 it isn't.

Author: scop

test/t/test_locale_gen.py

@@ -8,6 +8,6 @@ class TestLocaleGen:
     def test_1(self, completion):
         assert completion
 
-    @pytest.mark.complete("locale-gen --", require_cmd=True)
+    @pytest.mark.complete("locale-gen --", require_longopt=True)
     def test_2(self, completion):
         assert completion