ci/tests: generate grcov report

Signed-off-by: William Findlay<EMAIL_ADDRESS>

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@9ab29c6). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage        ?   29.39%
=======================================
  Files           ?       11
  Lines           ?      796
  Branches        ?      424
=======================================
  Hits            ?      234
  Misses          ?      255
  Partials        ?      307

:mega: Codecov can now indicate which changes are the most critical in Pull Requests. Learn more