Allow specifying a file name for `--coverage`

[whoabuddy]

To work around the OOM issue in #324 we split our tests so that they run separately in the CI.

Each one of the tests is run with --coverage and creates a generic coverage.lcov file. Right now we've added an extra step in the CI to rename this file after each test, but it would be much cleaner if we could just append an optional file name at the end.

clarinet test --coverage mia-coverage.lcov tests/cities/mia/