[mock-chronik-client] Return numTxs key in history() calls

Summary:
This is missing, although it is actually returned by chronik.

Since this key is to be used in a token-server feature, need it tested (ref D16615).

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16617