test: add structure field default value test

tests/lean/structDefault.lean

@@ -0,0 +1,11 @@
+structure A where
+  f : Nat → Nat → Nat
+
+structure B extends A where
+  f (a b : Nat) := 10
+
+structure C extends A where
+  f a b := 0
+
+structure D extends A where
+  f a b := true -- error

tests/lean/structDefault.lean.expected.out

@@ -0,0 +1,6 @@
+structDefault.lean:11:11: error: type mismatch
+  true
+has type
+  Bool
+but is expected to have type
+  Nat