[CI] Generate a CashTab coverage summary report

Summary:
This report is added to the coverage archive and will be used to add
CashTab coverage to the status panel.

Test Plan:

./contrib/teamcity/build-configurations.py cashtab-tests

Check that the coverage archive contains the coverage-summary.txt file

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8874