Fixed test failure on pypy

Code was working, just test was broken, due to placing test method
in wrong test class where UseConcatJoinMixin wasn't being used

tests/test_compiler.py

@@ -1137,6 +1137,45 @@ def foo(message_args, errors):
         )
         self.assertEqual(errs, [])
 
+    def test_variable_reuse_for_arg_lookup(self):
+        code, errs = compile_messages_to_python(
+            """
+            example = My name is { $name ->
+                [Peter] Peter11
+               *[other] Jane11
+              }
+              My gender is { $name ->
+                [Peter] Male
+               *[other] Female
+              }
+              """,
+            self.locale,
+        )
+        assert not errs
+        # $name should only be looked up once
+        self.assertCodeEqual(
+            code,
+            """
+            def example(message_args, errors):
+                try:
+                    _arg = message_args['name']
+                except (LookupError, TypeError):
+                    errors.append(FluentReferenceError('<string>:2:24: Unknown external: name'))
+                    _arg = FluentNone('name')
+                _plural_form = plural_form_for_number(_arg)
+                if _arg == 'Peter':
+                    _ret = 'Peter11'
+                else:
+                    _ret = 'Jane11'
+                _plural_form2 = plural_form_for_number(_arg)
+                if _arg == 'Peter':
+                    _ret2 = 'Male'
+                else:
+                    _ret2 = 'Female'
+                return 'My name is ' + _ret + '\\nMy gender is ' + _ret2
+        """,
+        )
+
     # TODO - eliminate unused assignments e.g. `_plural_form = plural_form_for_number(_arg)` is unneeded
     # def test_unused_assignments(self):
     #     code, errs = compile_messages_to_python("""
@@ -1278,41 +1317,3 @@ def test_non_unique_escaper(self):
             self.locale,
             escapers=[html_escaper, html_escaper],
         )
-
-    def test_variable_reuse_for_arg_lookup(self):
-        code, errs = self.compile_messages(
-            """
-            example = My name is { $name ->
-                [Peter] Peter11
-               *[other] Jane11
-              }
-              My gender is { $name ->
-                [Peter] Male
-               *[other] Female
-              }
-              """
-        )
-        assert not errs
-        # $name should only be looked up once
-        self.assertCodeEqual(
-            code,
-            """
-            def example(message_args, errors):
-                try:
-                    _arg = message_args['name']
-                except (LookupError, TypeError):
-                    errors.append(FluentReferenceError('<string>:2:24: Unknown external: name'))
-                    _arg = FluentNone('name')
-                _plural_form = plural_form_for_number(_arg)
-                if _arg == 'Peter':
-                    _ret = 'Peter11'
-                else:
-                    _ret = 'Jane11'
-                _plural_form2 = plural_form_for_number(_arg)
-                if _arg == 'Peter':
-                    _ret2 = 'Male'
-                else:
-                    _ret2 = 'Female'
-                return f'My name is {_ret}\\nMy gender is {_ret2}'
-        """,
-        )