test: avoid reusing AssumeUTXO coin insertion method in test fixtures #36601

Problem: Several coin-cache fixtures use EmplaceCoinInternalDANGER just to set up their coins, even though it is reserved for snapshot loading.

Fix: Use AddCoin and SpendCoin for fixture setup, keeping the entries dirty but not fresh, as before. Keep the simulation, fuzz and duplicate-insertion coverage that intentionally tests the bypass.

This is a small extraction from #36598 that can be reviewed and merged independently of the broader AssumeUTXO removal demo. The overlay fixture change also follows the earlier [suggestion in #35295](https://github.com/bitcoin/bitcoin/pull/35295#discussion_r3421846300).

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36601
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36601.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36601#author-ralph

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🚧 At least one of the CI tasks failed. _{Task 32 bit ARM: https://github.com/bitcoin/bitcoin/actions/runs/39741631849/job/119699974970} _{LLM reason (✨ experimental): CTest failed because ARM-target test executables could not run on the host and were interpreted as shell scripts, producing syntax errors.}

<details><summary>Hints</summary>

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