Add test of sync callbacks

[thunderbiscuit]

WIP. I'd like to add one for the FullScanScriptInspector as well.

Description

Notes to the reviewers

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

Changelog

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• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
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• This pull request breaks the existing API
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[thunderbiscuit]

Just putting this here to remember: our current implementation of the callback for sync is not perfect, because if you just want to know how many scripts are being synced, you need to do a bit of workaround to do it instead of just pulling the data out of the type. This is not an issue at the ffi layer but the Rust layer. Issue here: bitcoindevkit/bdk#2220.

Currently logging the number of scripts being synced for Tatooine with this (works but is clearly not great).

class SyncCallback : SyncScriptInspector {
    private val logger = KotlinLogging.logger {}
    var totalSynced = 0

    // On the first run of the callback, log the number of scripts that will be inspected.
    override fun inspect(script: Script, total: ULong) {
        if (totalSynced == 0) logger.info { "Syncing $total scripts in this sync" }
        totalSynced++
    }
}

Note that this triggers only once the sync is started, and so the number of scripts is unknown until the sync starts.