[stdlib] change errors in Array validation tests

The new replaceSubrange uses `UMBP.initialize(fromContentsOf:)`,
which raises its own fatalError, which is unfortunately less clear.
However, this is still a performance and codesharing win.

Author: oxy

validation-test/stdlib/Arrays.swift.gyb

@@ -1356,9 +1356,9 @@ final class EvilCollection : Collection {
 
 for (step, evilBoundsCheck) in [ (1, true), (-1, false), (-1, true) ] {
 
-  let message = step < 0 && evilBoundsCheck
-    ? evilBoundsError
-    : "invalid Collection: count differed in successive traversals"
+  let message = step < 0
+    ? "invalid Collection: count differed in successive traversals"
+    : "buffer cannot contain every element from source."
 
   let constructionMessage = step < 0
     ? "invalid Collection: less than 'count' elements in collection"