[Cashtab] Support testnet with env variable

Summary: Could be useful for some situations

Test Plan:
npm test succeeds if REACT_APP_TESTNET=false`

Update .env so that REACT_APP_TESTNET=TRUE

npm start

built site with this done is at https://670e5f1cdf65ca6e95978d34--cashtab-local-dev.netlify.app/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: tobias_ruck, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16896