CI: separate "normal" tests from coverage/junit-generating tests

[matt-codecov]

#12 failed a test but didn't get a test results comment because the later steps in the "Test" CI job were skipped on failure

this PR separates things into two test jobs:

• "Test", which just runs cargo test and prints the regular output to stdout
• "Test (Upload to Codecov)", which wraps cargo test in cargo llvm-cov and cargo2junit to emit coverage/test results and then uploads it all to Codecov

the second one does not print any test results to stdout, so if there are failures we will only be able to see them in the Codecov test results comment. if that comment is useful enough that we never need to look at the logs for the first one, then we can get rid of the first one

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (c0a4b65) to head (88b923f).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   99.21%   99.21%           
=======================================
  Files          17       17           
  Lines        7249     7249           
=======================================
  Hits         7192     7192           
  Misses         57       57           

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[Swatinem]

lgtm
Another idea would be to try cargo nextest, which can output junit automatically, and can also work together with llvm-cov. It does not support doctests though :-(