refactor: modernize typedef to using alias in test code #35805

This PR modernizes legacy typedef declarations to using aliases in the test suite (src/test/checkqueue_tests.cpp, src/test/transaction_tests.cpp, src/test/coins_tests.cpp).

This brings the test code in line with the modernization efforts applied to the rest of the codebase, adhering to modern C++ guidelines.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35805.

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Squash into #35799.