Move RegisterDecorator model into wagtail.tests.models

For some reason, registering it via a decorator as part of a function
call caused it to register itself in the wagtail.wagtailsnippets app,
which caused a false positive in the missing migrations check. Moving it
to the wagtail.tests.models file means it is registered in the correct
location.

wagtail/tests/migrations/0007_registerdecorator.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tests', '0006_merge'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='RegisterDecorator',
+            fields=[
+                ('id', models.AutoField(serialize=False, auto_created=True, verbose_name='ID', primary_key=True)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+    ]

wagtail/tests/models.py

@@ -497,3 +497,8 @@ class SnippetChooserModel(models.Model):
     panels = [
         SnippetChooserPanel('advert', Advert),
     ]
+
+
+@register_snippet
+class RegisterDecorator(models.Model):
+    pass

wagtail/wagtailsnippets/tests.py

@@ -4,7 +4,7 @@ from django.db import models
 
 from wagtail.tests.utils import WagtailTestUtils
 from django.test.utils import override_settings
-from wagtail.tests.models import Advert, AlphaSnippet, ZuluSnippet, SnippetChooserModel
+from wagtail.tests.models import Advert, AlphaSnippet, ZuluSnippet, SnippetChooserModel, RegisterDecorator
 from wagtail.wagtailsnippets.models import register_snippet, SNIPPET_MODELS
 
 from wagtail.wagtailsnippets.views.snippets import (
@@ -189,9 +189,6 @@ class TestSnippetRegistering(TestCase):
         self.assertIn(RegisterFunction, SNIPPET_MODELS)
 
     def test_register_function(self):
-        @register_snippet
-        class RegisterDecorator(models.Model):
-            pass
 
         # Misbehaving decorators often return None
         self.assertIsNotNone(RegisterDecorator)