[TEST] Handle legacy snapshots as if they don't exist anymore

An earlier commit removed BWC for pre-5.0 snapshots, which also meant removing the capability to load pre-5.0 snapshots. In 6.0, such snapshots are now
invisible and must be treated by the BWC tests in that way.

core/src/test/java/org/elasticsearch/bwcompat/RestoreBackwardsCompatIT.java

@@ -209,8 +209,7 @@ public class RestoreBackwardsCompatIT extends AbstractSnapshotIntegTestCase {
             client().admin().cluster().prepareRestoreSnapshot(repo, snapshot).setRestoreGlobalState(true).setWaitForCompletion(true).get();
             fail("should have failed to restore - " + repo);
         } catch (SnapshotRestoreException ex) {
-            assertThat(ex.getMessage(), containsString("cannot restore index"));
-            assertThat(ex.getMessage(), containsString("because it cannot be upgraded"));
+            assertThat(ex.getMessage(), containsString("snapshot does not exist"));
         }
     }
 }