Extract test coverage statistics from the gcov job

Extract test coverage statistics from the gcov job

In older GitLab versions, the regular expression used for extracting
test coverage statistics from the output of GitLab CI jobs was
configured in the project's settings, using GitLab's web interface.
That changed in recent GitLab versions [1]; the previous configuration
method was removed from the web interface altogether as of GitLab 15.0.
The relevant regular expression is now supposed to be set in the
relevant job's definition in .gitlab-ci.yml.

Set the regular expression used for extracting test coverage
statistics in the definition of the "gcov" GitLab CI job.  Use the
regular expression suggested in GitLab's documentation [2].

[1] https://docs.gitlab.com/ee/update/deprecations.html#test-coverage-project-cicd-setting
[2] https://docs.gitlab.com/ee/ci/pipelines/settings.html#test-coverage-examples