test: cover method return addition diagnostics

tests/typecheck_method_return_addition_bad.claro

@@ -0,0 +1,8 @@
+CLASS Player
+    HAS score NUMBER
+
+    TEACH total amount RETURNS NUMBER
+        CHECK TYPE amount IS NUMBER
+        RETURN amount + "oops"
+    END
+END

tests/typecheck_method_return_addition_good.claro

@@ -0,0 +1,11 @@
+CLASS Player
+    HAS score NUMBER
+
+    TEACH total amount RETURNS NUMBER
+        CHECK TYPE amount IS NUMBER
+        RETURN amount + 2
+    END
+END
+
+NEW Player player
+DO player.total 3