CI: make tarpaulin ignore test code

[msrd0]

Description

tarpaulin does not recognize the tests/ folder, async_std::test or tokio::test, so we'll ignore that code manually

also fixes a caching issue with GitHub actions

Checklist

• Formatted code using cargo fmt --all
• Linted code using clippy cargo clippy --all-targets --all-features -- -D warnings
• Updated README.md using cargo readme -r influxdb -t ../README.tpl > README.md
• Reviewed the diff. Did you leave any print statements or unnecessary comments?
• Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment

[msrd0]

Looks like I got it fixed for now. Hopefully tarpaulin gets smarter in the future about which code needs to be tested and what code is testing code.

[Empty2k12]

Great change, thanks!