Add support for --test-threads

[dianpopa]

We want to be able to specify to cargo-kcov the number of threads on which to run the tests.
We are currently facing an issue where we see some intermittent fails in our cargo kcov testing and they seem to be caused by multiple tests being run in parallel.
We have noticed that if we run cargo test --test-threads and then kcov on the binary, the number of failures is smaller.