[CI] Generate coverage reports for cashtab

Summary:
This diff adds coverage statistics to teamcity, and create an lcov html
report in the teamcity coverage report tab. Next step is the build
status panel integration.

Depends on D8842.

Test Plan:

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

Check this creates an abc-ci-builds/cashtab-tests/coverage.tar.gz
archive and prints out teamcity statistics commands.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8839