tests: silentpayments: deduplicate setup code #1972

Deduplicates code in silent payments as suggested by @theStack at #1943#pullrequestreview-5578778611.

• Turns output_tweak into a reusable constant.
• Adds function labels_cache_set_neg_output_tweak for setting the cache for negated OUTPUT_TWEAK.
• Adds function prevouts_summary_create_bob for creating secp256k1_silentpayments_prevouts_summary with bob's first address key.

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🟢 Approval recommended

No active review issues were identified for this test-only refactor consolidating shared data and setup helpers.

No open issues 🎉

<details> <summary><strong>What changed in this PR</strong></summary>

This PR reduces repeated setup in the silent payments tests by sharing test data and helper functions. All changes are confined to test code.

Changes:

• Extract and document the shared OUTPUT_TWEAK value.
• Add a helper for creating a prevouts summary with Bob’s first address key.
• Add a helper for populating the label cache with the negated output tweak.

</details>

_{Review effort: Max}

ACK 70f321c334dd0cfa0d8e074bc436094785bc3477

It's a bit counter-intuitive that in the recipient tests Bob's address key serves as the sender's input pubkey, while Alice's seckey is the scan key, i.e. the roles are flipped compared to the sender tests. Not a problem of this PR though, as that was already the case before; could be cleaned up when the tests are touched the next time.