Use caching to run tests and report coverage

[lspinheiro]

Why are these changes needed?

Currently, unit tests are running twice, once in the test step and a second time in code coverage, duplicating resource usage and increasing CI time. This PR updates the build process to always generate the the coverage file during the initial testing and cache it in GitHub Actions artifactions. During the coverage step we just download the files to measure coverage.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.91%. Comparing base (ce20de9) to head (fb5d31c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5086   +/-   ##
=======================================
  Coverage   69.91%   69.91%           
=======================================
  Files         168      168           
  Lines       10698    10698           
=======================================
  Hits         7479     7479           
  Misses       3219     3219           

Flag	Coverage Δ
unittests	69.91% <ø> (ø)

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[lspinheiro]

@jackgerrits @ekzhu , I think we are running out of resources in the worker to run tests in parallel. The autogen-ext tests are failing with The hosted runner: GitHub Actions 718 lost communication with the server. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.. Removing concurrency seems to fix the issue (I updated it to pytest -n 1 in autogen-ext/pyproject.toml in this test).

[jackgerrits]

Looks good, thank you!