test: deduplicate PSBT unknown-field coverage in `rpc_psbt.py` #36591

I've tested some mutations related to test coverage for PSBT unknown field merging and discovered an inconsistency.

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If line 71 of psbt.cpp "unknown.insert(psbt.unknown.begin(), psbt.unknown.end());" (inside bool PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt) function) is removed then, the expected test fails: "Test that combining PSBTs preserves unknown fields with the same and with distinct values per map."

If line 434 of psbt.cpp "unknown.insert(input.unknown.begin(), input.unknown.end());" (inside bool PSBTInput::Merge(const PSBTInput& input) function), is removed, a different unexpected test fails: "Test that unknown values are just passed through".

If line 537 of psbt.cpp "unknown.insert(output.unknown.begin(), output.unknown.end());" (inside bool PSBTOutput::Merge(const PSBTOutput& output) function), is removed, the expected test fails: "Test that combining PSBTs preserves unknown fields with the same and with distinct values per map.".

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This PR solves this inconsistency. The old code did 3 different things:

1. Test the pass-through of unknown values for walletprocesspsbt.
2. Combine two v0 PSBTs with different unknown input keys and check that both keys survive.
3. Combine implicit v0 PSBT with explicit v0 and check that both keys survive.

Case 2 is already fully covered by the existing test_combinepsbt_preseves_unknown_fields so keeping it is redundant and misleading.

Commit 40eba5eb20ba0041a6328ba3482fcc26ddbe0185 moves the old code into a new method called test_unknown_fields_passthrough and commit ef4310097bd651497a98a9d1bb59560a7055ae01 removes the redundant code of Case 2 and moves Case 3 inside test_combinepsbt_preserves_unknown_fields.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36591
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36591.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36591#author-ralph

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/39630407161/job/119326813663} _{LLM reason (✨ experimental): CI failed because lint checks found trailing whitespace on blank line 616 of test/functional/rpc_psbt.py, flagged by both Ruff and the trailing-whitespace check.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

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