Testing improvements on thirdweb Pay

We made a number of improvements on testing on thirdweb Pay.

1. Added a testMode property to the buyWithCrypto PayUIOptions. This will allow you to filter for all supported testnets and try out a crypto-to-crypto purchase experience without using mainnet funds.
2. Enabled Pay with Fiat on testnet purchases. No funds are delivered in this flow, but the credit card testnet experience will allow you to simulate an end-to-end user journey without requiring an actual credit card purchase.