[Cashtab] Prevent double notifications on send xec txs while preserving testing of component tx building

Summary:
Cashtab has better and standardized tx notifications that come from the websocket subscription. However, it is still important to test correct rawtxs are built in the component screens. We are not able to test the websocket tx notifications without regtest, but we can test the tx notifications generated by the components.

So, we conditionally render these notifications only in the test environment.

Test Plan: npm test, npm start and send some eCash, we do not see double notifications on send

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17544