Improve test for function call in groupchat

[sonichi]

Why are these changes needed?

The test #1243 misses await for the async test. This PR takes suggestions from @davorrunje to fix it and add check that the functions are indeed called.

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3b2955d) 31.60% compared to head (49430a1) 41.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1252      +/-   ##
==========================================
+ Coverage   31.60%   41.26%   +9.66%     
==========================================
  Files          32       32              
  Lines        4389     4389              
  Branches     1024     1079      +55     
==========================================
+ Hits         1387     1811     +424     
+ Misses       2892     2401     -491     
- Partials      110      177      +67     

Flag	Coverage Δ
unittests	41.19% <ø> (+9.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.