How to test payments in Facebook Instant Game?

Question

I cannot seem to make a test dialog appear. When I call payments.purchaseAync, I am always presented with a real Checkout dialog as opposed to a test dialog.

I've already added the test user to the Testers. Am I missing anything? Or is this feature not supported yet at this time?

Temporary code I'm using:

let supportedAPIs:any = FBInstant.getSupportedAPIs();
if(supportedAPIs.includes('payments.purchaseAsync'))
{
    console.log('payments supported...');

    FBInstant.payments.onReady(() => {
        console.log('payments ready...');

        FBInstant.payments.purchaseAsync({
            productID: 'test_product',
            developerPayload: 'foobar',
        }).then(function (purchase) {
            console.log(purchase);
        });
    });
}
else
{
    console.log('payments not supported...');
}