fix: avoid calling `GetCoin` and `SignTransaction()` inside of `assert(...)` in tests

[UdjinM6]

Technically, it's not possible to compile without assertions at the moment because of this check. However, if it would be removed one day then txpackage_tests will start to fail for such builds.

Running txpackage_tests compiled with the check removed and CPPFLAGS="-DNDEBUG": master: *** 21 failures are detected in the test module "Bitcoin Core Test Suite" this PR: *** No errors detected

[DrahtBot]

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #30849 (refactor: migrate bool GetCoin to return optional<Coin> by l0rinc)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[maflcko]

If you are worried about assert, you can use Assert, which does not have the "problem".

[DrahtBot]

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