Track code coverage

[radu-matei]

We should consider tracking code coverage for Spin.

A possible solution could be https://github.com/xd009642/tarpaulin and https://about.codecov.io.

[jdbohrman]

This is kinda relevant to #343 also

[lann]

I find tarpaulin + this extension pretty helpful.

[FrankYang0529]

I tried tarpaulin on Ubuntu 22.04. My command is cargo tarpaulin -v --follow-exec -t 600 --out html and it uses Ptrace engine to track the coverage. The coverage result is 24.47%. I will add it to the CI flow with codecov.io.

GitHub can't upload HTML files, so I add the extension .txt. tarpaulin-report.html.txt