Reimplement code coverage

[MikaelSlevinsky]

This used to work on Travis, but now that the free trial has been exhausted and the CI has mostly been migrated to GitHub Actions, the coverage flags make the tests extremely slow. If this can somehow be fixed, it may be reimplemented by the two environment variables COV=gcov and FT_COVERAGE=1, and post-success make coverage && bash <(curl -s https://codecov.io/bash).