[explorer] Update the url of the testnet faucet

Summary: I'm moving the faucet to a more manageable server and also updating the url while I'm at it.

Test Plan: It's just a URL change, the server is not deployed yet.

Reviewers: #bitcoin_abc, Kronkmeister

Reviewed By: #bitcoin_abc, Kronkmeister

Differential Revision: https://reviews.bitcoinabc.org/D16987