You can test bits in the rig using setUseLoopback(true); from the helper function. This way when you call useBits(SKU) it’ll simulate a transaction and call the callback you’ve set in onTransactionComplete(transaction => ...). The rig can’t currently simulate Twitch’s dialog box, so if you try set this to false you’ll run into issues when attempting to call useBits(SKU).
You can also test on a Twitch channel by setting setUseLoopback(false) and then when you call useBits(SKU) you’ll get Twitch’s dialog box asking you to confirm/cancel/buy bits. This can be done on a channel you’ve set as a test channel for your extension, even if it’s not an affiliate or partner.
It’s also worth noting that if your product SKU is set to in development then it wont actually subtract bits from a users account so that you can test the bits purchasing stuff without actually spending bits.