core: fix syntax of input-bad-var-default test fixture

This was incorrectly using block syntax to assign a value to the "default"
attribute.

terraform/test-fixtures/input-bad-var-default/main.tf

@@ -1,5 +1,5 @@
 variable "test" {
-    default {
+  default = {
-        l = [1, 2, 3]
+    l = [1, 2, 3]
-    }
+  }
 }