Test self referencing 'file'

When specifying the 'file' key to a value of it's own name, test
that this works and does not cause a false positive circular reference
error.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/fixtures/extends/specify-file-as-self.yml

@@ -0,0 +1,16 @@
+myweb:
+  extends:
+    file: specify-file-as-self.yml
+    service: web
+  environment:
+    - "BAR=1"
+web:
+  extends:
+    file: specify-file-as-self.yml
+    service: otherweb
+  image: busybox
+  environment:
+    - "BAZ=3"
+otherweb:
+  environment:
+    - "YEP=1"

tests/unit/config_test.py

@@ -413,6 +413,33 @@ class ExtendsTest(unittest.TestCase):
             },
         ])
 
+    def test_self_referencing_file(self):
+        """
+        We specify a 'file' key that is the filename we're already in.
+        """
+        service_dicts = load_from_filename('tests/fixtures/extends/specify-file-as-self.yml')
+        self.assertEqual(service_dicts, [
+            {
+                'environment':
+                {
+                    'YEP': '1', 'BAR': '1', 'BAZ': '3'
+                },
+                'image': 'busybox',
+                'name': 'myweb'
+            },
+            {
+                'environment':
+                {'YEP': '1'},
+                'name': 'otherweb'
+            },
+            {
+                'environment':
+                {'YEP': '1', 'BAZ': '3'},
+                'image': 'busybox',
+                'name': 'web'
+            }
+        ])
+
     def test_circular(self):
         try:
             load_from_filename('tests/fixtures/extends/circle-1.yml')