Add a simple test

Author: noti0na1

tests/explicit-nulls/pos/from-nullable.scala

@@ -0,0 +1,6 @@
+import scala.annotation.experimental
+
+@experimental def testFromNullable =
+  val s: String | Null = "abc"
+  val sopt1: Option[String] = Option(s) // error
+  val sopt2: Option[String] = Option.fromNullable(s) // ok