chore: retry database cleanup task

- Database cleanup task fails sporadically due to Deadline Exceeded error which happen due to various reasons including network issues they are safe to retry.
- This PR adds an exponential retry for the task.

spanner/spec/spec_helper.rb

@@ -74,10 +74,12 @@ def cleanup_instance_resources
   def cleanup_database_resources
     return unless @instance
 
-    @test_database = @instance.database @database_id
-    @test_database&.drop
-    @test_database = @instance.database @restored_database_id
-    @test_database&.drop
+    with_retry "cleanup database" do 
+      @test_database = @instance.database @database_id
+      @test_database&.drop
+      @test_database = @instance.database @restored_database_id
+      @test_database&.drop
+    end
   end
 
   def cleanup_backup_resources
@@ -247,4 +249,17 @@ def restore_database_from_backup
 
     @test_database
   end
+
+  def with_retry action, attempts: 5
+    @attempt_number ||= 0
+    yield
+    @attempt_number = nil
+  rescue Exception => e
+    @attempt_number += 1
+    puts "failed attempt #{@attempt_number} for #{action}"
+    sleep @attempt_number*@attempt_number
+    retry if @attempt_number < attempts
+    @attempt_number = nil
+    raise e
+  end
 end