Revert "fix: skip failing backup tests for now" (#634)

This reverts commit 990c612.

Author: thiagotnunes

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITDatabaseAdminTest.java

@@ -63,7 +63,6 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.ClassRule;
-import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.experimental.categories.Category;
 import org.junit.runner.RunWith;
@@ -270,7 +269,6 @@ public void onClose(Status status, Metadata metadata) {
  }
  }
 
- @Ignore("Skipping until list backup operations bug is fixed: b/169431286")
  @Test
  public void testRetryNonIdempotentRpcsReturningLongRunningOperations() throws Exception {
  assumeFalse(