Update test_coverage.py to allow running the tests for a single crate from workspace

[andreeaflorescu]

At the moment we're running the kcov command with --all flag. While this is a good default as we typically want to run all the tests in a workspace, there are situations in which we just want to run the tests for the current crate. This is the case for crates that we want to publish as part of the same workspace.