[Cashtab] Updating processChronikWsMsg unit test

Summary: The unit test for processChronikWsMsg is periodically breaking the build because even when the AddedToMempool event is picked up by the websocket, the fiatPrice input arg may not always be processed.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14192