chore(test): improve the start method of test_server

[caicancai]

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[caicancai]

@hubcio I improved the start method of test_server. I may not want to submit it in multiple PRs, which will be more troublesome for you, so I merged it into one PR

[hubcio]

could you please describe the motivation about this change in the PR description? why is it better? :)

[hubcio]

could you please improve it by checking if test server is alive in the method wait_until_server_has_bound()?