test: add legacy descriptor tests

[luisschwab]

Description

This PR closes #134 and is a revival of bitcoindevkit/bdk#1130, which adds the following tests:

• test_legacy_bump_fee_no_change_add_input_and_change()
• test_legacy_bump_fee_add_input()
• test_legacy_bump_fee_drain_wallet()
• test_legacy_bump_fee_zero_abs()
• test_legacy_create_tx_custom_sighash()
• test_legacy_create_tx_default_sighash()
• test_legacy_create_tx_absolute_high_fee()
• test_legacy_create_tx_absolute_zero_fee()
• test_legacy_create_tx_absolute_fee()
• test_legacy_create_tx_custom_fee_rate()
• test_legacy_get_funded_wallet_tx_fee_rate()

Modifications to the assert_fee_rate! macro were necessary for it to work with legacy transactions:

• Enable it to get the input amount from the legacy PSBT field.
• Drop the tx_fee_rate - fee_rate < half_default check for legacy transactions.

I also made the change descriptor optional on get_funded_wallet() as per bitcoindevkit/bdk#1533 and fixed clippy warnings.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[coveralls]

Pull Request Test Coverage Report for Build 14586112354

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 86.53%

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Totals
Change from base Build 14270666930:	0.005%
Covered Lines:	7272
Relevant Lines:	8404

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💛 - Coveralls