Fix nightly build by updating a named tuple test with language import (…

…scala#22059)

Because of the change from scala#22045, the test
`tests/pos/named-tuple-downcast.scala` from scala#22028 need to be updated
with `import scala.language.experimental.namedTuples`.

Fix scala#22058

tests/pos/named-tuple-downcast.scala

@@ -1,3 +1,5 @@
+import scala.language.experimental.namedTuples
+
 type Person = (name: String, age: Int)
 
 val Bob: Person = (name = "Bob", age = 33)