Fix check file

Is this a stability problem? It seems the CI visits Seq definitions in different order than local.

Author: odersky

tests/neg/i14966.check

@@ -2,8 +2,8 @@
 2 |  export s.* // error
   |           ^
   |           Double definition:
-  |           final def concat[B >: T](suffix: IterableOnce[B]): Set[B] in class B at line 2 and
-  |           final def concat(that: IterableOnce[T]): Set[T] in class B at line 2
+  |           final def ++[B >: T](suffix: IterableOnce[B]): Set[B] in class B at line 2 and
+  |           final def ++(that: IterableOnce[T]): Set[T] in class B at line 2
   |           have the same type after erasure.
   |
   |           Consider adding a @targetName annotation to one of the conflicting definitions