qt: fix 'opens in testnet mode when presented with a BIP-72 link with no fallback'
Passes tests:
$ ./bitcoin-qt 'bitcoin:?r=http://www.example.com/' .. fixed the original problem - this launches mainnet. $ ./bitcoin-qt 'bitcoin:mngeNQbTKnmaMbx8EXCYdwUbnt9JJD52cC' .. launches testnet $ ./bitcoin-qt -testnet 'bitcoin:1NXXeQRyMFFFRfyUix2o7mk1vhvk2Nxp78' .. sanity check - launches mainnet.
Fixes #4355. Closes #4411.