Test excluded bounds in `SpkTxOutIndex::outputs_in_range`

[notmandatory]

"For SpkTxOutIndex::outputs_in_range() the excluded bounds aren't tested, you can just change them to whatever and no test would fail."

[aagbotemi]

Hi @notmandatory, I raised a PR for this issue here bitcoindevkit/bdk#1897. Kindly help me take a look at it.