Set up test code coverage reporting as part of CI

[felixc]

The -Z option to codecov is required in order for it to exit with a non-success status code if there is an error, which we want in order to make the CI step fail if there's a problem.

This was set up using: https://www.collabora.com/news-and-blog/blog/2021/03/24/rust-integrating-llvm-source-base-code-coverage-with-gitlab/ https://doc.rust-lang.org/rustc/instrument-coverage.html https://github.com/mozilla/grcov

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@1df0ed5). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage        ?   35.88%           
=======================================
  Files           ?        1           
  Lines           ?      680           
  Branches        ?        0           
=======================================
  Hits            ?      244           
  Misses          ?      436           
  Partials        ?        0           

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