[TEST] Use actual target directory to fsync copied files in test

Apparently lucene6 is way more picky with respect to corrupting files
that are not fsynced that's why this test sometimes failed after the lucene6
upgrade.

core/src/test/java/org/elasticsearch/indices/recovery/RecoverySourceHandlerTests.java

@@ -94,7 +94,7 @@ public class RecoverySourceHandlerTests extends ESTestCase {
                     @Override
                     public void close() throws IOException {
                         super.close();
-                        store.directory().sync(Collections.singleton(md.name())); // sync otherwise MDW will mess with it
+                        targetStore.directory().sync(Collections.singleton(md.name())); // sync otherwise MDW will mess with it
                     }
                 };
             } catch (IOException e) {