Readd ExhibitB test with pickling exclusion

Author: aherlihy

compiler/test/dotc/pos-test-pickling.excludelist

@@ -71,6 +71,7 @@ i18211.scala
 named-tuples1.scala
 i20897.scala
 i20512.scala
+i22645b.scala
 
 # Opaque type
 i5720.scala

tests/pos/i22645b.scala

@@ -0,0 +1,12 @@
+import language.experimental.namedTuples
+object ExhibitE:  // works
+
+  type N = ("name", "age")
+  type Names = Tuple.Map[N, [X] =>> X]
+
+  class SelectableNT extends Selectable:
+    def selectDynamic(name: String) = ???
+    type Fields = NamedTuple.NamedTuple[Names, (String, Int)]
+
+  val x = new SelectableNT
+  x.name