ci: Upload coverage to Codecov in flagged stages for complete coverage report

[matthewfeickert]

Description

First noted in PR #1377, this allows for running the main tests, uploading the coverage to Codecov with a "unittests" flag, then running the tests for the contrib module, and uploading the new coverage report with these tests added to the main tests with a "contrib" flag.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Upload coverage reports to Codecov in staged runs with different flags to allow for iterative reporting
   - Allows for testing contrib module separately buy adding the coverage to the final report
   - c.f. https://docs.codecov.io/docs/flags

[matthewfeickert]

@lukasheinrich @kratsg This is a nice trick that we can probably use if we wanted finer coverage reporting in the future. 👍

[codecov]

Codecov Report

Merging #1380 (44c0d1b) into master (8a65295) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1380   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files          63       63           
  Lines        3808     3808           
  Branches      538      538           
=======================================
  Hits         3714     3714           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	24.18% <ø> (?)
unittests	97.53% <ø> (ø)

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