CI: Add automated test coverage reports. (#276)

* initial save state, need to experiment [ skip ci]

* try config

* more vars

* not that long URL

* json is not a host

* travis has to go

* canonic way of reporting

* coveralls+ way of reporting

* nightly explicitly

* tests with for cycle

* meant to compare coverages from loop and --workspace

* Update .gitlab-ci.yml

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

* test (ci): codecov with --workspace

* test (ci):

* test (ci): comparing test runs

* test (ci): old hack conflicts with a new one

* test (ci): -Zno-landing-pads caused test failures

* test (ci): try with cache

* test (ci): no cache and zip with tar

* test (ci): return to zip

* test (ci): no target caching, no release

* test (ci): better report

* test (ci): target dir var, cleanup after

* test (ci): final version

* test (ci): restore CI config

* change (ci): code coverage with grcov -> coveralls

* doc (ci): cleaner comments

* Update .gitlab-ci.yml

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

* Update .gitlab-ci.yml

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

* Update .gitlab-ci.yml

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

* Update .gitlab-ci.yml

Co-Authored-By: Hero Bird <robin.freyler@gmail.com>

Co-authored-by: Hero Bird <robin.freyler@gmail.com>
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