test_docker_commandline_code_executor.py : 161.66s -> 108.07 (#6429)

## 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](https://github.com/microsoft/autogen/commit/a3cf70bcf8017b9036dcf89b68ca0fa60b92a0fa)
[reuse executor at some tests
140s->120s](https://github.com/microsoft/autogen/commit/ca15938afa6253bd8db1b74d677f9bb427aed627)
[Remove unnecessary start of docker
120s->110s](https://github.com/microsoft/autogen/commit/61247611e01eff05c372e2bdc8b64c3a83c6d534)

## Related issue number

<!-- For example: "Closes #1234" -->
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.
- [x] I've made sure all auto checks have passed.

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>