test: do an initial coverage capture
    
    I noticed that our coverage reports miss some files completely - this
    happens when the test doesn't touch the code in them at all, so the
    generated coverage data (and resulting reports) have no information
    about them. Let's fix this by doing an initial zero coverage capture
    that contains a zeroed counter for every instrumented line in every
    file, so when we later merge it with a capture from the test, it shows up
    with a missing coverage instead of not showing at all.