test: Assumeutxo: ensure failure when importing a snapshot twice

[alfonsoromanz]

I am getting familiar with the assume_utxo tests and I found that the scenario of trying to activate a snapshot twice is not covered. This test is to ensure failure when loading a snapshot if there is already a snapshot-based chainstate.

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Type	Reviewers
ACK	fjahr, rkrux, kevkevinpal, achow101

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[fjahr]

test/functional/feature_assumeutxo.py:401:1: W293 blank line contains whitespace
^---- failure generated from lint-python.py

[fjahr]

Concept ACK

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_{Debug: https://github.com/bitcoin/bitcoin/runs/24300439204}

[alfonsoromanz]

test/functional/feature_assumeutxo.py:401:1: W293 blank line contains whitespace
^---- failure generated from lint-python.py

Thanks @fjahr. I just fixed the linting issue.

[fjahr]

Code review ACK b259b0e

[rkrux]

tACK b259b0e

Make successful, all functional tests pass.
Short and to the point PR.

[kevkevinpal]

tACK b259b0e

made a similar PR not knowing this existed #30068

[achow101]

ACK b259b0e