test: cover object field expressions

tests/typecheck_object_field_expression_bad.claro

@@ -0,0 +1,8 @@
+CLASS Player
+    HAS name TEXT
+    HAS score NUMBER
+END
+
+NEW Player player
+SET player.name "Ada"
+SET player.score player.name

tests/typecheck_object_field_expression_good.claro

@@ -0,0 +1,7 @@
+CLASS Player
+    HAS score NUMBER
+END
+
+NEW Player player
+SET player.score 10
+SET player.score player.score + 1