Test interpolation of keyword

Author: som-snytt

tests/neg-custom-args/fatal-warnings/i13440.scala

@@ -0,0 +1,7 @@
+import language.`3.0-migration`
+
+def given = 42 // error
+
+case class C(enum: List[Int] = Nil) { // error
+  val s = s"$enum" // error
+}