Support running test environments in parallel with `detox`

This patch namespaces the test temp directory and the output
coverage report file with the name of the environment under test,
such that separately run tests do not access the same files.

When running tests without tox, directly through setup.py,
then the tmp directory will still be `./tmp`.

  * .gitignore: Added new .coverage-reports/ directory

  * .gitlab-ci.yml: Rely on tox to combine and report coverage, only
    tell tox about the COVERAGE_PREFIX so that results can be namespaced
    by CI job name.

    This change also publishes the sources and final combined `.coverage`
    file in an output gitlab artifact for inspection, and lists some missing
    dependencies to the `coverage` job.

  * tox.ini: Add comments and refactor main [testenv] section so that
    other environments dont inherit too much unrelated cruft.

    Generate the coverate reports in the respective {envtmpdir} so that
    all per-process coverage files are prefixed with a full path, ensuring
    that concurrent runs don't mix reports and addressing concerns
    raised in #844.

    Also implemented new `tox -e coverage` environment to combine
    any found coverage and print a report.

Fixes issue #844