Add original test that caused the whole rewrite

Author: odersky

tests/pos/i3488.scala

@@ -0,0 +1,16 @@
+package test
+
+class Sett[A] {
+
+  def incl(elem: A): Sett[A] = ???
+
+  inline final def + (elem: A): Sett[A] = incl(elem)
+}
+
+object Sett {
+  def apply[A](elems: A*): Sett[A] = ???
+}
+
+class Test {
+  Sett(1) + 1
+}