Skip tests that depend on OpenAI via --skip-openai

[maxim-saplin]

Allows skipping test that depend on OpenAI. It is possible to run the whole test suite with openai dependency installed via pytest test --skip-openai (previously it was required to uninstall openai lib to run tests that don't interact with LLM).

Related issue number

Partially closes #1088

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.
• [N/A] 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 (f497710) 30.56% compared to head (e0cc7d1) 40.49%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1097      +/-   ##
==========================================
+ Coverage   30.56%   40.49%   +9.92%     
==========================================
  Files          30       30              
  Lines        4028     4033       +5     
  Branches      911      962      +51     
==========================================
+ Hits         1231     1633     +402     
+ Misses       2723     2289     -434     
- Partials       74      111      +37     

Flag	Coverage Δ
unittests	40.41% <ø> (+9.90%)	⬆️

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[qingyun-wu]

LGTM! Thanks!