test, assumeutxo: Add test to ensure failure when mempool not empty

[hernanmarino]

Add a test to ensure that loadtxoutset fails when the node's mempool is not empty, as suggested by maflcko here: https://github.com/bitcoin/bitcoin/pull/27596#discussion_r1344713537

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Type	Reviewers
ACK	maflcko, BrandonOdiwuor
Stale ACK	fjahr

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[maflcko]

lgtm ACK da13cc2c8010cbccf37324cca4403cb346ecdd30

Thanks

[fjahr]

utACK da13cc2c8010cbccf37324cca4403cb346ecdd30

I still think my suggestions here would be an improvement but are not strictly needed.

[hernanmarino]

utACK da13cc2

I still think my suggestions here would be an improvement but are not strictly needed.

Thanks for mentioning that, I think that leaving node 2 was a better alternative becuase it makes this a cleaner and more test , and avoids the errors that might arise in future code changes (mentioned by @maflcko and others that might arise) but i was struggling to find a solution and / or follow your suggestion somehow. It's nice that we can all agree in this way, thanks.

[hernanmarino]

Fixed the last nit: Pushed to remove the extra space mentioned by @fjahr https://github.com/bitcoin/bitcoin/compare/da13cc2c8010cbccf37324cca4403cb346ecdd30..8d20602e555dfe026b421363ee32cfca17c674d8

[maflcko]

Please don't invalidate two ACKs for a single space style issue. If you care about the style, it is your responsibility to take care of before opening the pull request, or before pushing the code. You can use any auto-formatter of your choice.

re-ACK 8d20602e555dfe026b421363ee32cfca17c674d8