There’s a big chance that the Japanese were going to surrender anyways, and if not, maybe just the threat of dropping the bomb (maybe, say, after a demonstration at sea or otherwise away from civilians) would’ve been enough.
They believe their Emperor was a God. The invasion of mainland Japan would have resulted in the Japanese fighting to the last man, woman, and child. Millions of civilian casualties. You suggest a test of the bomb would have forced Japanese surrender. But history tells the exact opposite story. There was a 3 day gap between the first and second bomb. Japanese high command thought the allies only had one bomb, refused to surrender. They only surrendered after the second bomb, when they realized this was repeatable.