test_docker_commandline_code_executor.py : 161.66s -> 108.07

[SongChiYoung]

Why are these changes needed?

Current autogen-ext's test is too slow.
So, I will search slow test case and makes more fast.

init docker executor function to module 180s->140s
reuse executor at some tests 140s->120s
Remove unnecessary start of docker 120s->110s

Related issue number

Part of #6376

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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 78.51%. Comparing base (bfdf816) to head (52daa96).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6429      +/-   ##
==========================================
- Coverage   78.54%   78.51%   -0.04%     
==========================================
  Files         225      225              
  Lines       16521    16521              
==========================================
- Hits        12977    12971       -6     
- Misses       3544     3550       +6     

Flag	Coverage Δ
unittests	78.51% <ø> (-0.04%)	⬇️

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