test: Generalise Empty Synth test

There isn't a strong need to test a full json payload. This avoids issues when
the cdktf version changes

tests/stacks/test_cally.py

@@ -8,4 +8,6 @@ class CallyStackTests(CallyTfTestHarness):
     def test_empty_synth(self):
         stack = CallyStack(service=self.empty_service)
         result = self.synth_stack(stack)
-        self.assertDictEqual(result, self.load_json_file('cdk/empty_synth.json'))
+        self.assertDictEqual(
+            result.get('terraform'), self.load_json_file('cdk/empty_synth.json')
+        )

tests/stacks/test_idp.py

@@ -20,4 +20,6 @@ def test_class_load(self):
     def test_empty_synth(self):
         stack = stacks.MinimalStack(service=self.empty_service)
         result = self.synth_stack(stack)
-        self.assertDictEqual(result, self.load_json_file('cdk/empty_synth.json'))
+        self.assertDictEqual(
+            result.get('terraform'), self.load_json_file('cdk/empty_synth.json')
+        )

tests/testdata/cdk/empty_synth.json

@@ -1,17 +1,7 @@
 {
-    "//": {
-        "metadata": {
-            "backend": "local",
-            "stackName": "test",
-            "version": "0.20.4"
-        },
-        "outputs": {}
-    },
-    "terraform": {
-        "backend": {
-            "local": {
-                "path": "state/test.tfstate"
-            }
+    "backend": {
+        "local": {
+            "path": "state/test.tfstate"
         }
     }
 }