CI: Improve coverage reporting

Closes #648 (closed)

Following Anton`s findings I modified our CI:

    - rm -f .coverage*
    - >
      export COV_CORE_SOURCE=: && export COV_CORE_CONFIG=: && export COV_CORE_DATAFILE=.coverage.eager
    # run full test suite with telemetry disabled (default), then re-run telemetry-specific tests with it enabled
    - >
      pytest --cov --cov-branch --cov-report=term --run_extra_src_tests --junitxml=report.xml ||
      pytest --lf --cov --cov-branch --cov-report=term --cov-append --run_extra_src_tests --junitxml=report-lf.xml
    - >
      TANGO_TELEMETRY_ENABLE=on pytest -k telemetry
      --cov --cov-branch --cov-report=term --cov-report=html --cov-report=xml --cov-append --junitxml=report-telem.xml
    # check typing stubs
    - python cmake/check_stubs.py