Storage NIO: Stop leaving leftover buckets in IT (#3898)

* Stop leaving leftover buckets in ITGcsNIO

This integration test now deletes all the buckets it creates.

The issue was that requester-pays buckets couldn't be deleted by the
existing code. This PR updates RemoteStorageHelper so it can be given a
userProject for requester-pays buckets.

* Don't set userProject if empty, add test

RemoteStorageHelper now doesn't set the userProject option if it's
empty. Also, added testForceDeleteRetriesWithUserProject unit test.

Author: jean-philippe-martin

java-storage-nio/google-cloud-nio/src/test/java/com/google/cloud/storage/contrib/nio/it/ITGcsNio.java

@@ -127,9 +127,13 @@ public static void beforeClass() throws IOException {
 
   @AfterClass
   public static void afterClass() throws ExecutionException, InterruptedException {
-    if (storage != null && !RemoteStorageHelper.forceDelete(storage, BUCKET, 5, TimeUnit.SECONDS) &&
-      log.isLoggable(Level.WARNING)) {
-        log.log(Level.WARNING, "Deletion of bucket {0} timed out, bucket is not empty", BUCKET);
+    if (storage != null) {
+      for (String bucket : new String[]{BUCKET, REQUESTER_PAYS_BUCKET}) {
+        if (!RemoteStorageHelper.forceDelete(storage, bucket, 5, TimeUnit.SECONDS, project) &&
+            log.isLoggable(Level.WARNING)) {
+          log.log(Level.WARNING, "Deletion of bucket {0} timed out, bucket is not empty", bucket);
+        }
+      }
     }
   }