[chronik-client] Git-ignore the coverage directory

Summary: It's generated when running tests.

Test Plan:

./contrib/teamcity/build-configurations.py chronik-client-integration-tests
git status

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16082