You can ‘simulate’ a transaction by using the setUseLoopback function: https://dev.twitch.tv/docs/extensions/bits/#setuseloopbackboolean
This way when you call useBits(sku) it’ll skip passing control over to Twitch which would normally open a dialog box to confirm/buy bits/cancel, and instead just call onTransactionComplete.
Also there’s nothing at all stopping you testing Bits outside of the rig by whitelisting a test channel, install the extension on that channel and test there, in which case you will have Twitch’s dialog box, and if the sku is set to being in development it wont actually subtract bits from your account (although you still do need sufficient bits in your account to get a confirm button, otherwise you just get options to cancel or buy bits)