test: Add coverage for unsatisfiable locktime combination in PSBT `ComputeTimeLock()` #36163

This PR adds a test case to psbt2_timelock_test covering an unsatisfiable locktime combination in PartiallySignedTransaction::ComputeTimeLock().

When different PSBT v2 inputs specify their own timelock requirement, ComputeTimeLock() needs to reconcile all of those into a single locktime for the whole transaction. To reconcile this locktime, all the inputs locktimes need to be height or time-based, but not a mix of them.

The existing test already covers this failure when the input #0 is height-based and a later input is time-based, returning std::nullopt. This PR adds the other case when the input #0 is time-based and a later input is height-based, returning std::nullopt. I've basically swapped the PSBT inputs from the already existing case to cover this one.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36163.

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ACK 69a640e05ec072c8bdce8d52dfd982df274e8262

Seems fine to add a bit coverage.