Allow running individual tests with coverage

[Pzixel]

There is a workaround currently to run individual tests and get coverage for them:

 cargo llvm-cov --lcov --output-path lcov.info  -- my::super::model::test_name --exact --nocapture

I think it is either worth documenting this as a recommended way of running individual tests or add an explicit option for this. Currently cargo-llvm-cov looks highly tuned for CI workflow while I think local development flow is also valuable, which you want to run one or a couple of tests at once, not the entire test suite.