[chronik] Test: make sure the tx reached the mempool before asserting on the ws message

Summary: This will help fixing/narrowing down the issue with the qemu machines missing the websocket message sometimes.

Test Plan:

./test/functional/test_runner.py chronik_ws_script

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14811