[chronik-client] Bump timeout for mocha tests

Summary:
Hitting the default 2s timeout about 30% of the time in these tests despite tests working as expected.

Bump timeout to 5s.

Test Plan: See D14915 instructions or wait for CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15342